C# as: Cast Examples

Use the as-cast. This cast helps with converting reference types.

As-keyword. We use "as" to cast. We gain performance and avoid exceptions when a cast is invalid. Null is returned when the cast is impossible.Casts

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.

An 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.
Info: On success, the newly-typed variable is available. In the program, variable3 is now a string.
And: We can use it just like any other string, like the original variable1. There is no difference.
Null: When you try an incompatible cast, such as with the StringBuilder type, the result of the casting expression is null.
C# program that uses as-casts using System; using System.Text; class Program { static void Main() { // Create a string variable and cast it to an object. string variable1 = "carrot"; object variable2 = variable1; // Try to cast it to a string. string variable3 = variable2 as string; if (variable3 != null) { Console.WriteLine("have string variable"); } // Try to cast it to a StringBuilder. StringBuilder variable4 = variable2 as StringBuilder; if (variable4 != null) { Console.WriteLine("have StringBuilder variable"); } } } Output have string variable

Example 2. The as-cast will not throw an exception if the cast fails, but if you do not check its result, your program will likely fail. Here we cast an object to a List—the cast fails.
Note: We try to use the null List, which causes an exception. A method cannot be called on a null instance.
Null ListNullReferenceException
C# program that causes NullReferenceException using System.Collections.Generic; class Program { static void Main() { string value = "cat"; object temp = value; List<string> list = temp as List<string>; list.Add("dog"); } } Output Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.

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.Is
Version 1: In this version of the code, we test an explicit reference type cast. We cast to the StringBuilder type from an object.
Version 2: Here we use the as-cast. We convert the object reference to a StringBuilder.
Result: In 2020, both casts perform about the same. So we can use whichever is clearest, or use the one with the best error handling.
C# program that times casting expressions using System; using System.Diagnostics; using System.Text; class Program { const int _max = 1000000000; static void Main() { 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")); } } Output 0.81 ns Cast 0.81 ns As-cast

FxCop. Excessive casting is a performance complaint in the FxCop static analysis tool. Also, the as-cast avoids throwing an exception on an invalid cast, which is much faster.

Internals. Next we consider the implementation of casts. At the instruction level, you will see that the as-cast is implemented in terms of the isinst opcode, the same used for the is-cast.
However: The isinst instruction is followed by further instructions. These store the local variable that was received by the ininst code.
And: By storing the local variable, the as-cast results in the overall reduction of cast instructions.

A summary. 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).

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