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Keep .cs in file-based app virtual .csproj file name #53182

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3135dd6
Keep `.cs` in file-based app virtual `.csproj` file name
jjonescz Feb 27, 2026
661d6b6
Preserve binary name when virtual project uses `.cs.csproj` extension
Copilot Mar 3, 2026
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3 changes: 3 additions & 0 deletions documentation/general/dotnet-run-file.md
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Original file line number Diff line number Diff line change
Expand Up @@ -65,6 +65,9 @@ Additionally, the implicit project file has the following customizations:
This avoids including files like `./**/*.resx` in simple file-based apps where users usually don't expect that.
See [multiple files](#multiple-files) for more details.

- `AssemblyName` is set to the entry-point file name without extension (e.g., `Program` for `Program.cs`)
to ensure the binary name is not affected by the virtual project file name (which includes the `.cs` extension).

## Grow up

When file-based programs reach an inflection point where build customizations in a project file are needed,
Expand Down
4 changes: 3 additions & 1 deletion src/Cli/dotnet/Commands/Run/Api/RunApiCommand.cs
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Expand Up @@ -75,7 +75,7 @@ public override RunApiOutput Execute()
builder.CreateProjectInstance(
new ProjectCollection(),
errorReporter,
out _,
out var project,
out var evaluatedDirectives,
validateAllDirectives: true);

Expand All @@ -91,6 +91,7 @@ public override RunApiOutput Execute()
return new RunApiOutput.Project
{
Content = csprojWriter.ToString(),
ProjectPath = project.FullPath,
Diagnostics = diagnostics.ToImmutableArray(),
};
}
Expand Down Expand Up @@ -163,6 +164,7 @@ public sealed class Error : RunApiOutput
public sealed class Project : RunApiOutput
{
public required string Content { get; init; }
public required string ProjectPath { get; init; }
public required ImmutableArray<SimpleDiagnostic> Diagnostics { get; init; }
}

Expand Down
6 changes: 4 additions & 2 deletions src/Microsoft.DotNet.ProjectTools/VirtualProjectBuilder.cs
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Expand Up @@ -85,7 +85,7 @@ internal static string GetArtifactsPath(string entryPointFileFullPath)
}

public static string GetVirtualProjectPath(string entryPointFilePath)
=> Path.ChangeExtension(entryPointFilePath, ".csproj");
=> entryPointFilePath + ".csproj";

/// <summary>
/// Obtains a temporary subdirectory for file-based app artifacts, e.g., <c>/tmp/dotnet/runfile/</c>.
Expand Down Expand Up @@ -375,7 +375,7 @@ ProjectRootElement CreateProjectRootElement(string projectFileText, ProjectColle
using var reader = new StringReader(projectFileText);
using var xmlReader = XmlReader.Create(reader);
var projectRoot = ProjectRootElement.Create(xmlReader, projectCollection);
projectRoot.FullPath = Path.ChangeExtension(EntryPointFileFullPath, ".csproj");
projectRoot.FullPath = GetVirtualProjectPath(EntryPointFileFullPath);
return projectRoot;
}
}
Expand Down Expand Up @@ -465,6 +465,7 @@ internal static void WriteProjectFile(
if (isVirtualProject)
{
Debug.Assert(!string.IsNullOrWhiteSpace(artifactsPath));
Debug.Assert(entryPointFilePath is not null);

// Note that ArtifactsPath needs to be specified before Sdk.props
// (usually it's recommended to specify it in Directory.Build.props
Expand All @@ -481,6 +482,7 @@ internal static void WriteProjectFile(
<FileBasedProgramsItemMapping>{CSharpDirective.IncludeOrExclude.DefaultMappingString}</FileBasedProgramsItemMapping>
<EnableDefaultCompileItems>false</EnableDefaultCompileItems>
<DisableDefaultItemsInProjectFolder>true</DisableDefaultItemsInProjectFolder>
<AssemblyName>{EscapeValue(Path.GetFileNameWithoutExtension(entryPointFilePath))}</AssemblyName>
""");

// Only set these to false when using the default SDK with no additional SDKs
Expand Down
33 changes: 29 additions & 4 deletions test/dotnet.Tests/CommandTests/Run/RunFileTests.cs
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Expand Up @@ -1903,6 +1903,23 @@ public void EmbeddedResource_AlongsideProj([CombinatorialValues("sln", "slnx", "
.And.HaveStdOut(considered ? "Resource not found" : "[MyString, TestValue]");
}

[Fact]
public void Restore_NonExistentPackage()
{
var testInstance = _testAssetsManager.CreateTestDirectory();
var programFile = Path.Join(testInstance.Path, "Program.cs");
File.WriteAllText(programFile, """
#:package Microsoft.ThisPackageDoesNotExist@1.0.0
Console.WriteLine();
""");

new DotnetCommand(Log, "restore", "Program.cs")
.WithWorkingDirectory(testInstance.Path)
.Execute()
.Should().Fail()
.And.HaveStdOutContaining("Program.cs.csproj : error NU1101");
}

[Fact]
public void NoRestore_01()
{
Expand Down Expand Up @@ -5520,6 +5537,7 @@ public void Api()
Console.WriteLine();
""");

var projectPath = VirtualProjectBuilder.GetVirtualProjectPath(programPath);
new DotnetCommand(Log, "run-api")
.WithStandardInput($$"""
{"$type":"GetProject","EntryPointFileFullPath":{{ToJson(programPath)}},"ArtifactsPath":"/artifacts"}
Expand All @@ -5539,6 +5557,7 @@ public void Api()
<FileBasedProgramsItemMapping>.cs=Compile;.resx=EmbeddedResource;.json=None;.razor=Content</FileBasedProgramsItemMapping>
<EnableDefaultCompileItems>false</EnableDefaultCompileItems>
<DisableDefaultItemsInProjectFolder>true</DisableDefaultItemsInProjectFolder>
<AssemblyName>Program</AssemblyName>
<OutputType>Exe</OutputType>
<TargetFramework>{ToolsetInfo.CurrentTargetFramework}</TargetFramework>
<ImplicitUsings>enable</ImplicitUsings>
Expand Down Expand Up @@ -5579,7 +5598,7 @@ public void Api()

</Project>

""")}},"Diagnostics":[]}
""")}},"ProjectPath":{{ToJson(projectPath)}},"Diagnostics":[]}
""");
}

Expand Down Expand Up @@ -5609,6 +5628,7 @@ public void Api_Evaluation()
var bPath = Path.Join(testInstance.Path, "B.cs");
File.WriteAllText(bPath, "");

var projectPath = VirtualProjectBuilder.GetVirtualProjectPath(programPath);
new DotnetCommand(Log, "run-api")
.WithStandardInput($$"""
{"$type":"GetProject","EntryPointFileFullPath":{{ToJson(programPath)}},"ArtifactsPath":"/artifacts"}
Expand All @@ -5628,6 +5648,7 @@ public void Api_Evaluation()
<FileBasedProgramsItemMapping>.cs=Compile;.resx=EmbeddedResource;.json=None;.razor=Content</FileBasedProgramsItemMapping>
<EnableDefaultCompileItems>false</EnableDefaultCompileItems>
<DisableDefaultItemsInProjectFolder>true</DisableDefaultItemsInProjectFolder>
<AssemblyName>Program</AssemblyName>
<EnableDefaultEmbeddedResourceItems>false</EnableDefaultEmbeddedResourceItems>
<EnableDefaultNoneItems>false</EnableDefaultNoneItems>
<OutputType>Exe</OutputType>
Expand Down Expand Up @@ -5667,7 +5688,7 @@ public void Api_Evaluation()

</Project>

""")}},"Diagnostics":[]}
""")}},"ProjectPath":{{ToJson(projectPath)}},"Diagnostics":[]}
""");
}

Expand All @@ -5681,6 +5702,7 @@ public void Api_Diagnostic_01()
#:property LangVersion=preview
""");

var projectPath = VirtualProjectBuilder.GetVirtualProjectPath(programPath);
new DotnetCommand(Log, "run-api")
.WithStandardInput($$"""
{"$type":"GetProject","EntryPointFileFullPath":{{ToJson(programPath)}},"ArtifactsPath":"/artifacts"}
Expand All @@ -5700,6 +5722,7 @@ public void Api_Diagnostic_01()
<FileBasedProgramsItemMapping>.cs=Compile;.resx=EmbeddedResource;.json=None;.razor=Content</FileBasedProgramsItemMapping>
<EnableDefaultCompileItems>false</EnableDefaultCompileItems>
<DisableDefaultItemsInProjectFolder>true</DisableDefaultItemsInProjectFolder>
<AssemblyName>Program</AssemblyName>
<EnableDefaultEmbeddedResourceItems>false</EnableDefaultEmbeddedResourceItems>
<EnableDefaultNoneItems>false</EnableDefaultNoneItems>
<OutputType>Exe</OutputType>
Expand Down Expand Up @@ -5734,7 +5757,7 @@ public void Api_Diagnostic_01()

</Project>

""")}},"Diagnostics":
""")}},"ProjectPath":{{ToJson(projectPath)}},"Diagnostics":
[{"Location":{
"Path":{{ToJson(programPath)}},
"Span":{"Start":{"Line":1,"Character":0},"End":{"Line":1,"Character":30}{{nop}}}{{nop}}},
Expand All @@ -5752,6 +5775,7 @@ public void Api_Diagnostic_02()
Console.WriteLine();
""");

var projectPath = VirtualProjectBuilder.GetVirtualProjectPath(programPath);
new DotnetCommand(Log, "run-api")
.WithStandardInput($$"""
{"$type":"GetProject","EntryPointFileFullPath":{{ToJson(programPath)}},"ArtifactsPath":"/artifacts"}
Expand All @@ -5771,6 +5795,7 @@ public void Api_Diagnostic_02()
<FileBasedProgramsItemMapping>.cs=Compile;.resx=EmbeddedResource;.json=None;.razor=Content</FileBasedProgramsItemMapping>
<EnableDefaultCompileItems>false</EnableDefaultCompileItems>
<DisableDefaultItemsInProjectFolder>true</DisableDefaultItemsInProjectFolder>
<AssemblyName>Program</AssemblyName>
<EnableDefaultEmbeddedResourceItems>false</EnableDefaultEmbeddedResourceItems>
<EnableDefaultNoneItems>false</EnableDefaultNoneItems>
<OutputType>Exe</OutputType>
Expand Down Expand Up @@ -5805,7 +5830,7 @@ public void Api_Diagnostic_02()

</Project>

""")}},"Diagnostics":
""")}},"ProjectPath":{{ToJson(projectPath)}},"Diagnostics":
[{"Location":{
"Path":{{ToJson(programPath)}},
"Span":{"Start":{"Line":0,"Character":0},"End":{"Line":1,"Character":0}{{nop}}}{{nop}}},
Expand Down
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