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Assigning out/ref parameters in Moq

Submitted by: @import:stackoverflow-api·Mar 10, 2026·

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moqrefoutparametersassigning

Problem

Is it possible to assign an out/ref parameter using Moq (3.0+)?

I've looked at using Callback(), but Action<> does not support ref parameters because it's based on generics. I'd also preferably like to put a constraint (It.Is) on the input of the ref parameter, though I can do that in the callback.

I know that Rhino Mocks supports this functionality, but the project I'm working on is already using Moq.

Solution

Moq version 4.8 and later has much improved support for by-ref parameters:

public interface IGobbler
{
    bool Gobble(ref int amount);
}

delegate void GobbleCallback(ref int amount);     // needed for Callback
delegate bool GobbleReturns(ref int amount);      // needed for Returns

var mock = new Mock();
mock.Setup(m => m.Gobble(ref It.Ref.IsAny))  // match any value passed by-ref
    .Callback(new GobbleCallback((ref int amount) =>
     {
         if (amount > 0)
         {
             Console.WriteLine("Gobbling...");
             amount -= 1;
         }
     }))
    .Returns(new GobbleReturns((ref int amount) => amount > 0));

int a = 5;
bool gobbleSomeMore = true;
while (gobbleSomeMore)
{
    gobbleSomeMore = mock.Object.Gobble(ref a);
}

The same pattern works for out parameters.

It.Ref.IsAny also works for C# 7 in parameters (since they are also by-ref).

Code Snippets

public interface IGobbler
{
    bool Gobble(ref int amount);
}

delegate void GobbleCallback(ref int amount);     // needed for Callback
delegate bool GobbleReturns(ref int amount);      // needed for Returns

var mock = new Mock<IGobbler>();
mock.Setup(m => m.Gobble(ref It.Ref<int>.IsAny))  // match any value passed by-ref
    .Callback(new GobbleCallback((ref int amount) =>
     {
         if (amount > 0)
         {
             Console.WriteLine("Gobbling...");
             amount -= 1;
         }
     }))
    .Returns(new GobbleReturns((ref int amount) => amount > 0));

int a = 5;
bool gobbleSomeMore = true;
while (gobbleSomeMore)
{
    gobbleSomeMore = mock.Object.Gobble(ref a);
}

Context

Stack Overflow Q#1068095, score: 211

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