MSDN Magazine - January 2008 - (Page 19)

Figure 4 Marshaling Results C# Signature Basic Types int arg out int arg ref int arg Structs MyStruct arg out MyStruct arg ref MyStruct arg Strings string arg out string arg ref string arg Classes MyClass arg MyClass *arg MyClass [in] MyClass out MyClass arg MyClass **arg [out] MyClass & [out] MyClass & char *arg char **arg char **arg string [out] string & string & [in] string [out] string & [in, out] string & MyStruct arg MyStruct *arg MyStruct *arg MyStruct [out] MyStruct & MyStruct & [in] MyStruct [out] MyStruct & [in, out] MyStruct & int arg int *arg int *arg int [out] int & int & [in] int [out] int & [in, out] int & Unmanaged Signature MSIL Signature Actual MSIL Signature Seen by the CLR PassPointerToComplexStructure( [In]ref ComplexStructure pStructure); The signature above will override the default directional behavior of ref, making it [InAttribute]only. In this particular case, if you are doing a P/Invoke, a pointer to Complex Structure (which is a value type) is being passed from the CLR side to the native side, but the callee can’t make any changes visible to the Com plexStructure pointed to by pStruc ture pointer. Figure 6 shows other examples of directional attributes and keyword combinations. So far we have only been discuss ing arguments. What about values returned from functions? The CLR ref MyClass arg MyClass **arg MyClass & [in, out] Myclass & automatically treats a return value as ior. A typical example of this default behavior is the case of an int if it is a normal argument using the [OutAttribute]. Also, the CLR argument; specifying [InAttribute] int arg isn’t necessary. can make the transformation to the function signature, a process There are cases when [OutAttribute] will be ignored. For exam controlled by the PreserveSigAttribute. If [PreserveSigAttribute] ple, [OutAttribute]int doesn’t make any sense, so the [OutAttribute] is set to false when applied to a P/Invoke signature, the CLR will is simply ignored by the CLR. The same is true of [OutAttribute] map HRESULT return values to managed exceptions, and it will string because string is immutable. map [out, retval] parameters to the function’s return value. So the Interface definition (IDL) files also have [in] and [out] attri following managed function signature public static string extern GetString(int id); butes, which can be considered the same as [InAttribute] and [Out would become the unmanaged signature: Attribute] in the CLR. HRESULT GetString([in]int id, [out, retval] char **pszString); Return Values Figure 5 Default Attributes Previously, we’ve shown that C# out and ref keywords can be di rectly mapped to [InAttribute] and [OutAttribute]. As a matter of C# Signature MSIL Signature Default Direction Attributes fact, out and ref can also change what data type the CLR will mar type [InAttribute] shal into or from. Passing data as out or ref is the same as passing out [OutAttribute] type & [OutAttribute] by reference. If you examine the corresponding function signature ref type & [InAttribute, OutAttribute] in intermediate language (IL) using ILDASM, you can see there is an ampersand character (&) next to the type, which means that the argument should be passed by ref Figure 6 More Attributes and Keywords erence. When passing by reference, an extra level Unmanaged IDL Signature MSIL Signature of indirection will be added by the CLR. Figure 4 C# Signature Out shows some examples. Compiler error CS0036. An out N/A Let’s summarize what we discussed for out and [InAttribute] out int arg parameter cannot have the In attribute. ref in the table shown in Figure 5. [OutAttribute] out int arg [out] int *arg [out] int & Please note that when passing by reference, if An N/A no directional attributes are specified, the CLR [InAttribute, OutAttribute] Compiler error CS0036.the out out int arg parameter cannot have In attribute. will apply [InAttribute] and [OutAttribute] au Ref tomatically, which is why there is only “string &” [InAttribute] ref int arg [in] int *arg [in] int & in the Microsoft Intermediate Language (MSIL) [OutAttribute] ref int arg Compiler error CS0662 cannot specify N/A signature in Figure 4. If any of these attributes are only Out attribute on a ref parameter. Use both In and Out attributes, or neither. specified, the CLR will follow them instead of the [InAttribute, OutAttribute] [in, out] int *arg [in] [out] int & default behavior, as shown in this example: public static extern void Keywords Out and Ref and Passing by Reference ref int arg CLR Inside Out january2008 19

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2008

MSDN Magazine - January 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Cutting Edge IIS 7.0 - Enhance Your Apps with the Integrated ASP.NET Pipeline World Ready - Around the World with ASP.NET AJAX Applications WCF Syndication - HTTP Programming with WCF and the .NET Framework 3.5 Wicked Code Foundations Extreme ASP.NET {End Bracket}

MSDN Magazine - January 2008

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