Check if value tuple is default

As of C# 7.3, tuple types now support == and !=. So your code could look like this:

(string foo, string bar) MyMethod() => default;

// Later
var result = MyMethod();
if (result == default){ } // Works!

See https://docs.microsoft.com/en-us/dotnet/csharp/tuples#equality-and-tuples

There are two problems with your attempts:

1. There is no == operator defined on tuples (in C# 7.2)
2. To get a default value for a tuple type, you need to parenthesize the type properly: default((int, int))

Note that an == operator is added to tuples in C# 7.3. Then you can do tuple == default (see live example).

If you really want to keep it returning default, you could use

result.Equals(default)

the built-in Equals method of a ValueTuple should work.

As of C# 7.3 value tuples now also support comparisons via == and != fully, Meaning you can now also do

result == default and it should work the same.

There are several ways of comparing default values to a value tuple:

    [TestMethod]
    public void Default()
    {
        (string foo, string bar) MyMethod() => default;
        (string, string) x = default;

        var result = MyMethod();

        // These from your answer are not compilable
        // Assert.IsFalse(x == default);
        // Assert.IsFalse(x == default(string string));
        // Assert.IsFalse(x is default);
        // Assert.IsFalse(x is default(string string));

        Assert.IsFalse(Equals(x, default));
        Assert.IsFalse(Equals(result, default));

        Assert.IsTrue(Equals(x, default((string, string))));
        Assert.IsTrue(Equals(result, default((string, string))));
        Assert.IsTrue(result.Equals(default));
        Assert.IsTrue(x.Equals(default));
        Assert.IsTrue(result.Equals(default((string, string))));
        x.Equals(default((string, string)))
    }

A simple defaultbefore it's used in a comparison must be reified from its "pure" null to a value tuple with default values for the members.

Here's what I have under the debugger: