Mocking library with the perfect balance of Performance and Intuitive API

🧘Balanced Performance and Developer Experience

Imposter delivers top-tier performance without sacrificing a natural, intuitive API. While being ~10× faster than NSubstitute and up to ~50× faster than Moq in common mocking scenarios, Imposter is not the fastest mocking library available. Instead, it focuses on achieving a balance between performance and API ease of use.

🚀 Quick Start

Add nuget package reference:

dotnet add package Imposter

Pick a target

namespace Application.Domain;

public interface ICalculator
{
    int Add(int a, int b);
}

Use [GenerateImposter] attribute in your tests project, this will generate an imposter

[assembly: GenerateImposter(typeof(Application.Domain.ICalculator))]

Use the generated imposter

using System.Threading.Tasks;
using Imposter.Abstractions;

// c# 14
var imposter = ICalculator.Imposter();

// c# 9 - 13
// var imposter = new ICalculatorImposter();

imposter.Add(Arg<int>.Any(), Arg<int>.Any())
    .Returns(1)
    .Then()
    .Returns(2);

var calculator = imposter.Instance();

calculator.Add(1, 2); // 1
calculator.Add(1, 2); // 2

🛡️ Strongly typed all the way

Loosely typed callbacks can easily cause runtime exceptions. Imposter enforces strong typing across the entire mocking pipeline, eliminating type mismatch errors for good. See more here

🎭 Interfaces and classes

Not limited to interfaces — Imposter can impersonate non-sealed classes and their protected members. See more here

🧬 Full generic support

Generics are fully supported. see more here

🧩 Mock any member

Imposter supports not only methods, but also properties, events and indexers.

🔀 Implicit and Explicit Modes

Choose how unmocked members behave. Implicit mode returns defaults silently — great for prototyping. Explicit mode throws on any call without a setup — ideal for unit tests that must be precise. See more here

🧵 Thread-safe by design

Imposter is built to work reliably in multi-threaded and parallel test environments.

⏱️ Benchmark

Square method is setup to return input * input and ran it for 1, 10, 100, and 1000 iterations. see benchmarks for more details

public interface ICalculator
{
    int Square(int input);
}

Method	Iteration	Mean	Allocated
Moq	1	78,776.14 ns	13148 B
NSubstitute	1	1,624.16 ns	7664 B
FakeItEasy	1	1,939.11 ns	5617 B
Rocks	1	37.91 ns	304 B
Imposter	1	188.72 ns	2408 B
Moq	10	1,153,399.52 ns	117789 B
NSubstitute	10	11,919.78 ns	27897 B
FakeItEasy	10	9,464.72 ns	35727 B
Rocks	10	627.42 ns	1888 B
Imposter	10	1,712.15 ns	22424 B
Moq	100	7,488,213.33 ns	1445159 B
NSubstitute	100	205,099.09 ns	222676 B
FakeItEasy	100	172,337.20 ns	732299 B
Rocks	100	25,495.06 ns	17728 B
Imposter	100	30,005.13 ns	222584 B
Moq	1000	98,890,235.56 ns	43233637 B
NSubstitute	1000	19,440,858.58 ns	2174740 B
FakeItEasy	1000	15,170,287.96 ns	46895901 B
Rocks	1000	1,868,905.85 ns	176128 B
Imposter	1000	2,256,458.40 ns	2224184 B

Benchmark Environment

BenchmarkDotNet v0.15.6, Windows 11 (10.0.26200.6899)
13th Gen Intel Core i9-13900HX 2.20GHz, 1 CPU, 32 logical and 24 physical cores
.NET SDK 10.0.100
[Host]     : .NET 8.0.21 (8.0.21, 8.0.2125.47513), X64 RyuJIT x86-64-v3
DefaultJob : .NET 8.0.21 (8.0.21, 8.0.2125.47513), X64 RyuJIT x86-64-v3

Benchmarks on macOS (Apple M2 Pro)

Method	Iteration	Execution Time	Allocated Memory
Moq	1	62,199.5 ns	12.85 KB
NSubstitute	1	1,688.9 ns	8.05 KB
FakeItEasy	1	2,023.3 ns	5.68 KB
Rocks	1	46.2 ns	0.30 KB
Imposter	1	250.8 ns	2.73 KB
Moq	10	634,227.3 ns	115.07 KB
NSubstitute	10	9,484.3 ns	28.46 KB
FakeItEasy	10	9,346.8 ns	35.06 KB
Rocks	10	587.9 ns	1.84 KB
Imposter	10	2,419.2 ns	25.65 KB
Moq	100	6,619,086.5 ns	1412.43 KB
NSubstitute	100	224,316.0 ns	236.98 KB
FakeItEasy	100	228,507.4 ns	715.71 KB
Rocks	100	28,261.9 ns	17.31 KB
Imposter	100	36,118.9 ns	254.87 KB
Moq	1000	98,252,350.2 ns	42221.29 KB
NSubstitute	1000	25,223,566.8 ns	2312.93 KB
FakeItEasy	1000	19,298,298.4 ns	45797.90 KB
Rocks	1000	2,574,515.3 ns	172.00 KB
Imposter	1000	4,011,942.7 ns	2547.05 KB

Benchmark Environment

BenchmarkDotNet v0.15.6, macOS Sequoia 15.7.3 (24G419) [Darwin 24.6.0]
Apple M2 Pro, 1 CPU, 10 logical and 10 physical cores
.NET SDK 10.0.101
[Host]     : .NET 10.0.1 (10.0.1, 10.0.125.57005), Arm64 RyuJIT armv8.0-a
DefaultJob : .NET 10.0.1 (10.0.1, 10.0.125.57005), Arm64 RyuJIT armv8.0-a

✨ Feature-Rich

• Method Impersonation
• Property Impersonation
• Indexer Impersonation
• Event Impersonation
• Class Impersonation
• Generics
• Implicit & Explicit Modes
• Use Base implementation
• Async Support
• Protected members Impersonation

📜 MIT License

Free to use, modify, and distribute under the MIT License.