Poxing a mocked generic method with Moq

Question

I provide a generic provider for Json serialization:

public interface IJsonSerializerProvider
{   
    T? Deserialize<T>(string value);

    string Serialize(object? obj);

    object? Deserialize(string value, string type);    
}

public class JsonSerializerProvider : IJsonSerializerProvider
{ ... }

On my tests, I provide a mocked version of this one which proxying real provider as default feature like this:

  object? paramObj = null;

  JsonSerializerProviderMock
  .Setup(x => x.Serialize(IsAny<object?>()))
  .Callback<object?>(s =>
  {
      paramObj = s;
  })
  .Returns<object?>(x => new JsonSerializerProvider().Serialize(paramObj));

   var param1 = "";
   var param2 = "";

   JsonSerializerProviderMock
  .Setup(x => x.Deserialize<IsAnyType>(IsAny<string>()))
  .Callback<string>(s =>
  {
      param1 = s;
  })
  .Returns<IsAnyType>(x => new JsonSerializerProvider().Deserialize<IsAnyType>(param1));

   JsonSerializerProviderMock
  .Setup(x => x.Deserialize(IsAny<string>(), IsAny<string>()))
  .Callback<string, string>((s, r) =>
  {
      param1 = s;
      param2 = r;
  })
  .Returns<object?>(x => new JsonSerializerProvider().Deserialize(param1, param2));

Unfortunately, I got this error at startup for the last definition:

System.ArgumentException
Invalid callback. Setup on method with 2 parameter(s) cannot invoke callback with different  number of parameters (1).
   at Moq.MethodCall.ValidateNumberOfCallbackParameters(Delegate callback, MethodInfo callbackMethod)

Are you sure that doing all that configuring is simpler and easier to read than just creating a regular class for mocking? And for what purpose do you want to insert a proxy? — JonasH
– JonasH, Commented Oct 8 at 9:58
Try replacing <IsAnyType> with the actual type your test uses (e.g. string). — PMF
– PMF, Commented Oct 8 at 10:47
My wetware compiler can't compile the example. Please edit the question so that it provides a minimal, reproducible example. — Mark Seemann
– Mark Seemann, Commented Oct 8 at 10:53

Gopalakrishnan R · Accepted Answer · 2025-10-09 07:19:34Z

0

the issue was on the Returns as it is having a wrong signature.

below code should work.

 object? paramObj = null;
 Mock<IJsonSerializerProvider> JsonSerializerProviderMock = new Mock<IJsonSerializerProvider>();

 var param1 = "";
 var param2 = "";

 JsonSerializerProviderMock
.Setup(x => x.Deserialize(IsAny<string>(), IsAny<string>()))
.Callback<string, string>((s, r) =>
{
    param1 = s;
    param2 = r;
})
.Returns(new JsonSerializerProvider().Deserialize(param1, param2));

answered Oct 9 at 7:19

Gopalakrishnan R

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