Expression bodied Function Weird Behavior

Question

When I am using

    var frontPage = await GetFrontPage();

    protected override async Task<WordDocument> GetFrontPage()
    {
        return null;
    }

This code works fine and I am getting null value in frontpage variable. but when I am rewriting the function as

protected override Task<WordDocument> GetFrontPage() => null;

I am getting an NullReferenceException.

Could anyone help me to understand the difference between the two statements.?

Answer 1

Could anyone help me to understand the difference between the two statements.?

Your first declaration is async, so the compiler generates appropriate code to make it return a Task<WordDocument> which has a result with the result of the method. The task itself is not null - its result is null.

Your second declaration is not async, therefore it just returns a null reference. Any code awaiting or otherwise-dereferencing that null reference will indeed cause a NullReferenceException to be thrown.

Just add the async modifier to the second declaration and it'll work the same as the first.

Note that there are no lambda expressions here - your second declaration is an expression-bodied method. It just uses the same syntax (=>) as lambda expressions.

Answer 2

try with:

Func<Task<WordDocument>> frontPage = async () => await GetFrontPage();

protected override async Task<WordDocument> GetFrontPage() => await Task.FromResult<WordDocument>(null);

Answer 3

This works for me:

    protected override async Task<WordDocument> GetFrontPage() => await Task.FromResult<WordDocument>(null);