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The In and Out Attributes
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The .NET marshaler always copies a parameter s values from .NET to the unmanaged DLL, but it copies the value back only if the parameter is passed by reference. This means that you see the new value if the callee changes the value of a ByRef parameter. At first glance, therefore, the semantics of the ByVal and ByRef keywords are preserved when working with PInvoke and COM Interop. However, things aren t always so smooth, and you must be prepared to work around a few potential problems you might have with reference types. As I explained earlier, the PInvoke marshaler can pass a reference type only if the type is a formatted class (that is, it uses an explicit layout), with String and StringBuilder types being among the few exceptions. The PInvoke marshaler copies the fields of a formatted structure to unmanaged memory instead of just passing a pointer to the managed object. (The COM Interop marshaler, however, simply passes the object pointer, as you d expect.)
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Part VII:
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As you know, if you pass a ByVal object to a Visual Basic .NET routine that modifies the object s fields, you see the new fields values when the call returns. Conversely, the PInvoke marshaler doesn t copy data back to the object when you pass a reference type to an external DLL, to achieve better performance. In other words, by default, ref erence types are passed as input-only arguments. You can change this behavior by flagging the argument with explicit In and Out attributes, as follows:
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In is a reserved word, so you can t omit the Attribute portion. Declare Sub TestRoutine Lib mydll (<InAttribute, Out> objref As Object)
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The StringBuilder class is an exception to this rule because the PInvoke marshaler cop ies it back even if you pass it by value and without the Out attribute. You can omit the In attribute when passing a ByRef argument that doesn t have to be initialized; in some circumstances, this omission can save a few CPU cycles when passing the object to a COM object in another apartment. In its quest for greater efficiency, under certain circumstances the marshaler can decide to pin the passed argument, a detail that makes things a bit more complicated. When an argument is pinned, the marshaler passes the address of the original value in the managed heap rather than the address of a copy. (The word pinning is used because the object is pinned in memory and doesn t move even if a garbage collection occurs in the meantime.) Data can be pinned in both PInvoke calls and intra-apartment COM Interop calls. For example, under PInvoke, a .NET string passed by value to a Unicode string argu ment is pinned: the callee receives the address of the internal character buffer, and no copy occurs. If the callee doesn t abide by the by-value semantics and mistakenly changes the passed string, the managed heap might be corrupted. (This happens, for example, if a longer string is assigned.) Pinning occurs also when you pass a blittable formatted class because the marshaler can improve performance by passing a pointer to the .NET object s memory in the managed heap. Pinning happens regardless of whether you use the Out attribute. (In other words, the Out attribute forces a copy of the value back to .NET only if the argu ment isn t pinned.) Pinning can occur only with strings passed by value. The PInvoke marshaler always copies ByRef string arguments to a temporary buffer and then passes the address of this buffer to the external routine. When the call returns, the marshaler creates a new string with the characters found in the buffer and passes the new string back to the caller. (This double copy is necessary to preserve .NET string immutability.) Arrays are reference types and can be pinned too. More precisely, an array is pinned if its elements are blittable and you make either a PInvoke call or a COM Interop call to an object that lives in the same apartment as the caller. Arrays defined as part of a structure are dealt with as other objects, but they can be passed by value by flagging them with a MarshalAs attribute, as follows:
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30:
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<StructLayout(LayoutKind.Sequential)> Structure MyStruct <MarshalAs(UnmanagedType.ByValArray, SizeConst:=64)> _ Public s1() As Short End Structure
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