any value type

Task<TResult>

void

any reference type

Task

The caller of a void-returning async method can't await it

If the method with async modifier doesn't contain an await expression or statement the compiler will generate a compilation error.

Without an await expression or statement a method with async modifier will execute synchronously.

async method can declare in, ref and out parameters

An async method runs asynchronously until it reaches its first await expression

With Task

With any type

Only in async context

With Task<T>

In catch or finally blocks

With a method which has await in its body

With any methods

With any lambdas and anonymous methods which return Task

With classes

With any method which returns Task

Abstract classes cannot have constructors.

An abstract class can have operator overloading.

Abstract classes can only contain abstract methods.