I am writing unit tests in ASP.NET Core using xUnit and Moq, and mocking StackExchange.Redis.

While setting up a mock for IConnectionMultiplexer.GetDatabase(), I get the following compiler error:

CS0854: An expression tree may not contain a call or invocation that uses optional arguments

The error occurs on this line:

mockConn.Setup(c => c.GetDatabase()).Returns(mockDb.Object);

Why this error occurs

The method IConnectionMultiplexer.GetDatabase() is defined as:

IDatabase GetDatabase(int db = -1, object asyncState = null);

This method has optional parameters.

• Moq uses expression trees

• Expression trees do NOT support optional arguments

• Calling GetDatabase() without explicitly passing all parameters is not allowed inside Setup(...)