How to typecast data pointed by void pointer to a fixed size int array in C#?

You can simply cast it to an int* pointer.. hoping for the best.. obviously:

// unsafe {}

var a = stackalloc int[5];
a[0] = 1;
a[1] = 2;
a[2] = 3;
a[3] = 4;
a[4] = 5;

var v = (void*) a; // cast to void as example

for (var i = 0; i < 5; i++)
    Console.WriteLine((*(int*)v)++); // back to int - 1, 2, 3, 4, 5

That said.. you will have to be vary careful with bounds checking. AFAIK there is no direct translation that allows for bounds.

I'm not entirely sure that this is what you're looking for, but one example would be like this:

int[] items = new int[10];

unsafe
{
    fixed ( int* pInt = &items[0] )
    {
        // At this point you can pass the pointer to other functions
        // or read/write memory using it.
        *pInt = 5;
    }
}

When taking the address of an array, you have to take the address of the first item in the array - hence &items[0] in the example above.

If you receive the pointer as a void* function parameter, you have to cast it inside the function:

public static unsafe void F ( void* pMem )
{
    int* pInt = (int*) pMem;

    // Omitted checking the pointer here, etc. This is something
    // you'd have to do in a real program.
    *pInt = 1;
}

If you receive a void* from an external source, you'll have to somehow know how many bytes (or ints, etc.) are safe to access through the pointer. The data may be delimited by a special value (like a terminating 0 or something else) or you'd need a count or bytes / elements to safely access memory through the pointer.

Update

Here's an example of calling an unmanaged function implemented in C:

// Function declaration in C
#define EXPORTFUNC __declspec(dllexport)
#define MYDLLAPI __declspec(nothrow) WINAPI

EXPORTFUNC int MYDLLAPI MyFunc1 ( byte* pData, int nDataByteCount );

// Import function in C#
[DllImport ( "My.dll" )]
private static extern int MyFunc1 ( byte* pData, int nDataByteCount );

// Call function with data (in unsafe class / method)
byte[] byData = GetData ( ... ); // returns byte array with data

fixed ( byte* pData = byData )
{
    int nResult = MyFunc1 ( pData, byData.Length );

    ...
}

MSDN has more examples on various pointer operations. Also, here's another article about marshaling arrays.