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Enable building in-memory projects (#12156)
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‎documentation/specs/build-nonexistent-projects-by-default.md

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# BuildNonexistentProjectsByDefault Global Property
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## Summary
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The `_BuildNonexistentProjectsByDefault` global property enables MSBuild tasks to build in-memory or virtual projects by defaulting to `SkipNonexistentProjects=Build` behavior when the property is not explicitly specified.
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## Background and Motivation
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### Problem
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[File-based applications][file-based-apps] (such as `dotnet run file.cs`) create in-memory MSBuild projects without corresponding physical `.csproj` files on disk. When these projects use common targets that include MSBuild tasks referencing the current project (e.g., `<MSBuild Projects="$(MSBuildProjectFullPath)" />`), the build fails because MSBuild cannot find the project file on disk, even though the project content is available in memory.
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This pattern is very common in .NET SDK targets, creating friction for file-based applications that need to reuse existing build logic.
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### Use Case Example
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Consider a file-based application that creates an in-memory project:
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```csharp
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var xmlReader = XmlReader.Create(new StringReader(projectText));
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var projectRoot = ProjectRootElement.Create(xmlReader);
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projectRoot.FullPath = Path.Join(Environment.CurrentDirectory, "test.csproj");
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// Project exists in memory but not on disk
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```
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When this project uses targets containing:
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```xml
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<MSBuild Projects="$(MSBuildProjectFullPath)" Targets="SomeTarget" />
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```
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The build fails with:
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> MSB3202: The project file "test.csproj" was not found.
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## Solution
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### The `_BuildNonexistentProjectsByDefault` Property
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This internal global property provides an opt-in mechanism to change the default behavior of MSBuild tasks when `SkipNonexistentProjects` is not explicitly specified.
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**Property Name:** `_BuildNonexistentProjectsByDefault`
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**Type:** Boolean
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**Default:** `false` (when not set)
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**Scope:** Global property only
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### Behavior
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When `_BuildNonexistentProjectsByDefault` is set to `true`:
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1. **MSBuild tasks** that don't explicitly specify `SkipNonexistentProjects` will default to `SkipNonexistentProjects="Build"` instead of `SkipNonexistentProjects="False"`
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2. **In-memory projects** with a valid `FullPath` can be built even when no physical file exists on disk
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3. **Existing explicit settings** are preserved - if `SkipNonexistentProjects` is explicitly set on the MSBuild task, that takes precedence
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### Implementation Details
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The property is checked in two MSBuild task implementations:
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1. **`src/Tasks/MSBuild.cs`** - The standard MSBuild task implementation
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2. **`src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs`** - The backend intrinsic task implementation
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The logic follows this precedence order:
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1. If `SkipNonexistentProjects` is explicitly set on the MSBuild task → use that value
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2. If `SkipNonexistentProjects` metadata is specified on the project item → use that value
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3. If `_BuildNonexistentProjectsByDefault=true` is set globally → default to `Build`
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4. Otherwise → default to `Error` (existing behavior)
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## Usage
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### File-based Applications
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File-based applications can set this property when building in-memory projects:
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```csharp
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var project = ObjectModelHelpers.CreateInMemoryProject(projectContent);
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project.SetGlobalProperty("_BuildNonexistentProjectsByDefault", "true");
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bool result = project.Build();
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```
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### SDK Integration
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The .NET SDK will use this property to enable building file-based applications without workarounds when calling MSBuild tasks that reference the current project.
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## Breaking Changes
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**None.** This is an opt-in feature with an internal property name (prefixed with `_`). Existing behavior is preserved when the property is not set.
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## Alternatives Considered
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1. **Always allow building in-memory projects**: This would be a breaking change as it could mask legitimate errors when projects are missing.
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2. **Add a new MSBuild task parameter**: This would require modifying all existing targets to use the new parameter, creating compatibility issues.
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3. **Modify SkipNonexistentProjects default**: This would be a breaking change affecting all MSBuild usage.
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4. **Engine-level configuration**: More complex to implement and would require serialization across build nodes.
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The global property approach provides the needed functionality while maintaining backward compatibility and requiring minimal changes to the MSBuild task implementations.
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## Related Issues
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- [#12058](https://github.com/dotnet/msbuild/issues/12058) - MSBuild task should work on virtual projects
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- [dotnet/sdk#49745](https://github.com/dotnet/sdk/pull/49745) - Remove MSBuild hacks for virtual project building
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- [NuGet/Home#14148](https://github.com/NuGet/Home/issues/14148) - Related workaround requirements
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- [File-based app spec][file-based-apps] - Motivating use-case
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[file-based-apps]: https://github.com/dotnet/sdk/blob/main/documentation/general/dotnet-run-file.md

‎src/Build.UnitTests/BackEnd/MSBuild_Tests.cs

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File.Delete(projectFile2);
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}
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}
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[Fact]
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public void InMemoryProject_Build()
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{
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Project project = ObjectModelHelpers.CreateInMemoryProject("""
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<Project>
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<Target Name="Build">
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<MSBuild Projects="$(MSBuildProjectFullPath)" Targets="Other" SkipNonexistentProjects="Build" />
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</Target>
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<Target Name="Other">
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<Message Text="test message from other" />
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</Target>
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</Project>
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""");
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var logger = new MockLogger();
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bool result = project.Build(logger);
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_testOutput.WriteLine(logger.FullLog);
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Assert.True(result);
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logger.AssertLogContains("test message from other");
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}
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[Fact]
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public void InMemoryProject_Error()
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{
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Project project = ObjectModelHelpers.CreateInMemoryProject("""
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<Project>
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<Target Name="Build">
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<MSBuild Projects="$(MSBuildProjectFullPath)" Targets="Other" SkipNonexistentProjects="False" />
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</Target>
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<Target Name="Other">
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<Message Text="test message from other" />
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</Target>
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</Project>
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""");
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var logger = new MockLogger();
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bool result = project.Build(logger);
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_testOutput.WriteLine(logger.FullLog);
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Assert.False(result);
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logger.AssertLogDoesntContain("test message from other");
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logger.AssertLogContains("MSB3202"); // error MSB3202: The project file was not found.
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}
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/// <summary>
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/// This is used by file-based apps (<c>dotnet run file.cs</c>) which use in-memory projects
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/// and want to support existing targets which often invoke the <c>MSBuild</c> task on the current project.
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/// </summary>
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[Fact]
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public void InMemoryProject_BuildByDefault()
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{
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Project project = ObjectModelHelpers.CreateInMemoryProject("""
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<Project>
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<Target Name="Build">
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<MSBuild Projects="$(MSBuildProjectFullPath)" Targets="Other" />
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</Target>
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<Target Name="Other">
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<Message Text="test message from other" />
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</Target>
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</Project>
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""");
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project.SetGlobalProperty(PropertyNames.BuildNonexistentProjectsByDefault, bool.TrueString);
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var logger = new MockLogger();
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bool result = project.Build(logger);
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_testOutput.WriteLine(logger.FullLog);
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Assert.True(result);
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logger.AssertLogContains("test message from other");
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}
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[Theory]
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[InlineData(null)]
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[InlineData(false)]
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[InlineData(true)]
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public void NonExistentProject(bool? buildNonexistentProjectsByDefault)
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{
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Project project = ObjectModelHelpers.CreateInMemoryProject("""
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<Project>
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<Target Name="Build">
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<MSBuild Projects="non-existent-project.csproj" Targets="Other" />
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</Target>
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<Target Name="Other">
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<Message Text="test message from other" />
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</Target>
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</Project>
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""");
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if (buildNonexistentProjectsByDefault is { } b)
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{
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project.SetGlobalProperty(PropertyNames.BuildNonexistentProjectsByDefault, b.ToString());
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}
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var logger = new MockLogger();
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bool result = project.Build(logger);
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_testOutput.WriteLine(logger.FullLog);
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Assert.False(result);
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logger.AssertLogDoesntContain("test message from other");
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logger.AssertLogContains(buildNonexistentProjectsByDefault == true
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? "MSB4025" // error MSB4025: The project file could not be loaded.
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: "MSB3202"); // error MSB3202: The project file was not found.
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}
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}
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}

‎src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs

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{
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skipNonExistProjects = behavior;
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}
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else if (BuildEngine is IBuildEngine6 buildEngine6 && buildEngine6.GetGlobalProperties()
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.TryGetValue(PropertyNames.BuildNonexistentProjectsByDefault, out var buildNonexistentProjectsByDefault) &&
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ConversionUtilities.ConvertStringToBool(buildNonexistentProjectsByDefault))
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{
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skipNonExistProjects = SkipNonExistentProjectsBehavior.Build;
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}
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else
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{
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skipNonExistProjects = SkipNonExistentProjectsBehavior.Error;

‎src/Shared/Constants.cs

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internal const string TargetFrameworks = nameof(TargetFrameworks);
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internal const string TargetFramework = nameof(TargetFramework);
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internal const string UsingMicrosoftNETSdk = nameof(UsingMicrosoftNETSdk);
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/// <summary>
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/// When true, `SkipNonexistentProjects=Build` becomes the default setting of MSBuild tasks.
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/// </summary>
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internal const string BuildNonexistentProjectsByDefault = "_" + nameof(BuildNonexistentProjectsByDefault);
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}
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// TODO: Remove these when VS gets updated to setup project cache plugins.

‎src/Tasks/MSBuild.cs

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{
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skipNonExistProjects = behavior;
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}
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else if (BuildEngine is IBuildEngine6 buildEngine6 && buildEngine6.GetGlobalProperties()
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.TryGetValue(PropertyNames.BuildNonexistentProjectsByDefault, out var buildNonexistentProjectsByDefault) &&
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ConversionUtilities.ConvertStringToBool(buildNonexistentProjectsByDefault))
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{
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skipNonExistProjects = SkipNonExistentProjectsBehavior.Build;
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}
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else
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{
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skipNonExistProjects = SkipNonExistentProjectsBehavior.Error;

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