The "is" and "as" in C#

The is operator checks if an object can be cast to a specific type.

Example:

if (someObject is StringBuilder) ...

The as operator attempts to cast an object to a specific type, and returns null if it fails.

Example:

StringBuilder b = someObject as StringBuilder;
if (b != null) ...

Also related

The cast operator attempts to cast an object to a specific type, and throws an exception if it fails.

Example:

StringBuilder b = (StringBuilder)someObject.