Getting Started¶

Imposter — The Fastest and Most Memory-Efficient Mocking Library

Installation¶

PackageReference.NET CLIPackage Manager

<ItemGroup>
    <PackageReference Include="Imposter" Version="*" PrivateAssets="all" />
</ItemGroup>

dotnet add package Imposter

Install-Package Imposter

This package includes both the source generator (analyzer) and the runtime abstractions.

Pro tip

To inspect generated sources locally, set EmitCompilerGeneratedFiles and CompilerGeneratedFilesOutputPath in your project. Remove these before committing.

<!-- Source code generation settings-->
<PropertyGroup>
    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
    <CompilerGeneratedFilesOutputPath>GeneratedFiles</CompilerGeneratedFilesOutputPath>
</PropertyGroup>

<!-- Include but don't compile generated code. The system will already compile the code, but this will make it visible for reference.-->
<ItemGroup>
    <Compile Remove="GeneratedFiles\**"/>
    <None Include="GeneratedFiles\**"/>
</ItemGroup>

Generate an imposter for an interface¶

Annotate the target type with the assembly level attribute and build. The generator produces a <TypeName>Imposter you can new up in code.

Example

using Imposter.Abstractions;

[assembly: GenerateImposter(typeof(IMyService))]

public interface IMyService
{
    int Increment(int value);
}

Note

Imposter and the imposter target (type that you generate an impsoter for) does not have to live in the same assembly. You can generate imposters for types in referenced assemblies.

After a build, use the generated type:

C# 14C# 9-13

var imposter = IMyService.Imposter();
imposter.Increment(Arg<int>.Any()).Returns(3);

var service = imposter.Instance();
service.Increment(1).ShouldBe(3);

Use the generated imposter type directly:

var imposter = new IMyServiceImposter();
imposter.Increment(Arg<int>.Any()).Returns(3);

var service = imposter.Instance();
service.Increment(1).ShouldBe(3);

Generate imposter for a class

Warning

Only non-sealed classes can be impersonated.

In the exact same fashion, you can generate imposter for classes.

Example

using Imposter.Abstractions;

[assembly: GenerateImposter(typeof(BaseService))]

public abstract class BaseService
{
    public virtual int GetNumber() => 0;
}

After a build, use the generated type:

C# 14C# 9-13

var imposter = BaseService.Imposter();
imposter.GetNumber().Returns(42);

var service = imposter.Instance();
service.GetNumber().ShouldBe(42);

var imposter = new BaseServiceImposter();
imposter.GetNumber().Returns(42);

var service = imposter.Instance();
service.GetNumber().ShouldBe(42);

Note

For classes, only virtual or abstract members can be impersonated.

Warning

Minimum supported C# version is 9.0

Next steps