As-keyword

In C# programs we use the as-keyword to cast. We gain performance and avoid exceptions when a cast is invalid. Null is returned when the cast is impossible.

For reference types, the as-cast is recommended. It is both fast and safe. We can test the resulting variable against null and then use it. This eliminates extra casts.

Example

Here we cast a string to the base object class type. Then the object variable is changed to a string with the as-operator.

using System;
using System.Text;

string c = "carrot";
object o = c;

// Part 1: try to cast the object to a string.
string s = o as string;
if (s != null)
{
    Console.WriteLine("have string variable");
}

// Part 2: try to cast it to a StringBuilder.
StringBuilder b = o as StringBuilder;
if (b != null)
{
    Console.WriteLine("have StringBuilder variable");
}
have string variable

Null

If we do not check the result of an as-cast, our program may fail due to a null reference. Here we cast an object to a List. We try to use the null value, which causes an exception.

using System.Collections.Generic;

string value = "cat";
object temp = value;
List<string> list = temp as List<string>;
// Exception because the variable is null.
list.Add("dog");
Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at ...\Program.cs:line 7

Is, pattern matching

In modern times we can use an "is type" pattern-matching cast in an if-statement instead of as-cast. This captures the casted variable, so we can use it within the if-block.

using System;

string c = "abc";
object x = c;

// Use "is" pattern-matching in if-statement instead of an as-cast.
if (x is string e)
{
    Console.WriteLine("Length of string is {0}", e.Length);
}
Length of string is 3

Benchmark, casts

The as-cast is sometimes the most efficient cast. We can use the variable afterwards. There is seldom any reason to use an is-cast before an as-cast.

using System;
using System.Diagnostics;
using System.Text;

const int _max = 1000000000;
StringBuilder builder = new StringBuilder();
object item = (object)builder;

// Version 1: use cast.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    StringBuilder build = (StringBuilder)item;
    if (build == null)
    {
        throw new Exception();
    }
}
s1.Stop();
// Version 2: use as-cast.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    StringBuilder build = item as StringBuilder;
    if (build == null)
    {
        throw new Exception();
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
0.81 ns    Cast
0.81 ns    As-cast

With "as" we cast reference variables. This cast receives 2 operands: the variable to cast, and the type you are trying to cast to. It returns the cast variable (or null).