Blittable Types - Blittable Types Defined

Blittable Types Defined

A blittable type is a data type that does not require special attention from the interop marshaler because by default it has a common representation in managed and unmanaged memory. By pinning the data in memory, the garbage collector will be prevented from moving it, allowing it to be shared in-place with the unmanaged application. This means that both managed and unmanaged code will alter the memory locations of these types in a consistent manner, and much less effort is required by the marshaler to maintain data integrity. The following are some examples of blittable types available in the .NET framework:

System.Byte
System.SByte
System.Int16
System.UInt16
System.Int32
System.UInt32
System.Int64
System.IntPtr
System.UIntPtr

Additionally, one-dimensional arrays of these types (including unsafe fixed buffers) as well as complex types containing only instance fields (which includes readonly fields) of these types are blittable. The presence of static or const fields that are non-blittable does not cause the type to become non-blittable, because such fields play no part in marshalling. Complex types (that is structs or classes) must also have instance field layout of Sequential applied using the attribute in order to be considered blittable by the .net marshaler. Structs have this attribute applied automatically by the compiler, but you must explicitly add it to a class definition if you want an otherwise blittable class to be blittable.

If a type is not one of the blittable types, then it is classified as non-blittable. The reason a type is considered non-blittable is that for one representation in managed memory, it may have several potential representations in unmanaged memory or vice-versa. Alternatively, there may be exactly one representation for the type in both managed and unmanaged memory. It is also often the case that there simply is no representation on one side or the other. The following are some commonly-used non-blittable types in the .NET framework:

System.Boolean
System.Char
System.Object
System.String

There are many more blittable and non-blittable types, and user-defined types may fit in either category depending on how they are defined (MSDN).

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