A string to be persisted in the build environment, or null if the System.Web.Compilation.BuildProvider instance does not implement BuildProvider.GetCustomString(System.CodeDom.Compiler.CompilerResults).
The BuildManager.GetCompiledCustomString(string) method of a System.Web.Compilation.BuildManager object creates an instance of a build provider for an input virtual path, and calls the BuildProvider.GetCustomString(System.CodeDom.Compiler.CompilerResults) method on the System.Web.Compilation.BuildProvider object.
The base System.Web.Compilation.BuildProvider class returns null for the BuildProvider.GetCustomString(System.CodeDom.Compiler.CompilerResults) method, indicating that BuildProvider.GetCustomString(System.CodeDom.Compiler.CompilerResults) is not implemented.
Certain build provider implementations can implement BuildProvider.GetCustomString(System.CodeDom.Compiler.CompilerResults) in place of the BuildProvider.GetGeneratedType(System.CodeDom.Compiler.CompilerResults) method. A System.Web.Compilation.BuildProvider object typically uses BuildProvider.GetGeneratedType(System.CodeDom.Compiler.CompilerResults) to return a Type object for a virtual path. However, the BuildProvider.GetCustomString(System.CodeDom.Compiler.CompilerResults) method can be used to indicate that a System.Web.Compilation.BuildProvider object can generate multiple types from a virtual path.