String.IsInterned Method
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Retrieves a reference to a specified String.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- str
- Type: System.String
The string to search for in the intern pool.
Return Value
Type: System.StringA reference to str if it is in the common language runtime intern pool; otherwise, null.
| Exception | Condition |
|---|---|
| ArgumentNullException | str is null. |
The common language runtime automatically maintains a table, called the "intern pool", which contains a single instance of each unique literal string constant declared in a program, as well as any unique instance of String you add programmatically.
The intern pool conserves string storage. If you assign a literal string constant to several variables, each variable is set to reference the same constant in the intern pool instead of referencing several different instances of String that have identical values.
This method looks up str in the intern pool. If str has already been interned, a reference to that instance is returned; otherwise, null is returned.
Important Note: |
|---|
This method does not return a Boolean value. If you call the method because you want a Boolean value that indicates whether a particular string is interned, you can use code such as the following. |
using System; public class Example { public static void Demo(System.Windows.Controls.TextBlock outputBlock) { string str1 = "a"; string str2 = str1 + "b"; string str3 = str2 + "c"; string[] strings = { "value", "part1" + "_" + "part2", str3, String.Empty, null }; foreach (var value in strings) { if (value == null) continue; bool interned = String.IsInterned(value) != null; if (interned) outputBlock.Text += String.Format("'{0}' is in the string intern pool.\n", value); else outputBlock.Text += String.Format("'{0}' is not in the string intern pool.\n", value); } } } // The example displays the following output: // 'value' is in the string intern pool. // 'part1_part2' is in the string intern pool. // 'abc' is not in the string intern pool. // '' is in the string intern pool.
Compare this method to the Intern method.
The following code example demonstrates that literal strings are interned automatically by the compiler.
// Sample for String.IsInterned(String) using System; using System.Text; using System.Runtime.CompilerServices; // In the .NET Framework 2.0 the following attribute declaration allows you to // avoid the use of the interning when you use NGEN.exe to compile an assembly // to the native image cache. [assembly: CompilationRelaxations(CompilationRelaxations.NoStringInterning)] class Example { public static void Demo(System.Windows.Controls.TextBlock outputBlock) { // String str1 is known at compile time, and is automatically interned. String str1 = "abcd"; // Constructed string, str2, is not explicitly or automatically interned. String str2 = new StringBuilder().Append("wx").Append("yz").ToString(); outputBlock.Text += "\n"; Test(outputBlock, 1, str1); Test(outputBlock, 2, str2); } public static void Test(System.Windows.Controls.TextBlock outputBlock, int sequence, String str) { outputBlock.Text += String.Format("{0}) The string, '", sequence); String strInterned = String.IsInterned(str); if (strInterned == null) outputBlock.Text += String.Format("{0}', is not interned.", str) + "\n"; else outputBlock.Text += String.Format("{0}', is interned.", strInterned) + "\n"; } } //This example produces the following results: //1) The string, 'abcd', is interned. //2) The string, 'wxyz', is not interned. //If you use NGEN.exe to compile the assembly to the native image cache, this //example produces the following results: //1) The string, 'abcd', is not interned. //2) The string, 'wxyz', is not interned.
Important Note: