Setup a method on a mock which has a parameter which is an async function expression

Question

I have a relatively simple interface to mock for which i want to setup the Run method:

public interface IRunner
{
    T Run<T>(ILogger log, ExecutionContext context, Func<FunctionContext, T> func);
}

When it gets invoked it looks something like:

myRunner.Run(myLog, myContext, async ctx => {
  await _do(myContext, myLog, trxData);
});

I have been trying to mock it this way:

var mockRunner = new Mock<IRunner>();
mockRunner.Setup(
  x => x.Run(
    It.IsAny<ILogger>,
    It.IsAny<ExecutionContext>,
    It.IsAny<Func<FunctionContext,T>>
  ));

But the compiler is not happy with that:

The type arguments for method 'IRunner.Run<T>(ILogger,ExecututionContext,Func<FunctionContext,T>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

I'm completely stumped on this and very grateful for any ideas or advice.

Answer 1

Run method of the IRunner interface is generic, and the type T, you need to provide explicit type arguments to the method you're setting up.

I've assumed that T is Task since you mentioned using await in the lambda passed to Run.

It.IsAny<Func<FunctionContext, Task>>(), specifies a function that takes a FunctionContext and returns a Task.

For example, if Run should return a Task, you should set up the mock to return an instance of Task

mockRunner.Setup(
  x => x.Run<Task<ResultType>>(
    It.IsAny<ILogger>(),
    It.IsAny<ExecutionContext>(),
    It.IsAny<Func<FunctionContext, Task<ResultType>>>()
  )
).ReturnsAsync(new ResultType());