What's the point of “As” keyword in C#

Question

From the docs:

The as operator is like a cast except that it yields null on conversion failure instead of raising an exception. More formally, an exp

Answer 1

Efficiency and Performance

Part of performing a cast is some integrated type-checking; so prefixing the actual cast with an explicit type-check is redundant (the type-check occurs twice). Using the as keyword ensures only one type-check will be performed. You might think "but it has to do a null check instead of a second type-check", but null-checking is very efficient and performant compared to type-checking.

if (x is SomeType )
{
  SomeType y = (SomeType )x;
  // Do something
}

makes 2x checks, whereas

SomeType y = x as SomeType;
if (y != null)
{
  // Do something
}

makes 1x -- the null check is very cheap compared to a type-check.