NSubstitute - mock out parameter behaviour for any parameter

configProvider.TryGet("key1", out Arg.Any<string>()) is not valid C# syntax, which is why it wont compile.

You need to use an actual variable for the out parameter.

The following works when tested.

//Arrange            
var expectedResult = true;
var expectedOut = "42";
var actualOut = "other";
var anyStringArg = Arg.Any<string>();
var key = "key1";
var configProvider = Substitute.For<IConfigurationProvider>();
configProvider
    .TryGet(key, out anyStringArg)
    .Returns(x => {
        x[1] = expectedOut;
        return expectedResult;
    });

//Act
var actualResult = configProvider.TryGet(key, out actualOut);

//Assert
Assert.AreEqual(expectedOut, actualOut); // PASS.
Assert.AreEqual(expectedResult, actualResult); // PASS.

As of NSubstitute 4+ this is supported out the box:

Matching out and ref args

---

Argument matchers can also be used with out and ref (NSubstitute 4.0 and later with C# 7.0 and later).

calculator
    .LoadMemory(1, out Arg.Any<int>())
    .Returns(x => {
        x[1] = 42;
        return true;
    });

var hasEntry = calculator.LoadMemory(1, out var memoryValue);
Assert.AreEqual(true, hasEntry); Assert.AreEqual(42, memoryValue); 

Source

Make sure you note the argument index used above (x[1] = 42;), this array includes the input and output variables but you can only set the value of an out variable.