You can apply this attribute to parameters.

The System.Runtime.InteropServices.InAttribute is optional. The attribute is supported for COM interop and platform invoke only. In the absence of explicit settings, the interop marshaler assumes rules based on the parameter type, whether the parameter is passed by reference or by value, and whether the type is blittable or non-blittable. For example, the System.Text.StringBuilder class is always assumed to be In/Out and an array of strings passed by value is assumed to be In.

You cannot apply the System.Runtime.InteropServices.InAttribute to a parameter modified with the C#-styled out keyword. To avoid confusing the In keyword in Visual Basic with the System.Runtime.InteropServices.InAttribute, minus Attribute, use the <[In]> form with brackets around the attribute.

Combining the System.Runtime.InteropServices.InAttribute and System.Runtime.InteropServices.OutAttribute is particularly useful when applied to arrays and formatted, non-blittable types. Callers see the changes a callee makes to these types only when you apply both attributes. Since these types require copying during marshaling, you can use System.Runtime.InteropServices.InAttribute and System.Runtime.InteropServices.OutAttribute to reduce unnecessary copies.

For more information on the effect of System.Runtime.InteropServices.InAttribute on marshaling behavior, see [<topic://cpcondirectionalattributes>].