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How to Resolve Error "CS0242: The operation in question is undefined on void pointers" in C#

The Compiler Error CS0242 is a pointer arithmetic error that occurs in Unsafe Code. The message reads: "The operation in question is undefined on void pointers".

In C#, pointer arithmetic (like ptr++, ptr + 1, or ptr - 1) depends entirely on the size of the data type being pointed to.

  • If you increment an int*, the memory address increases by 4 bytes (size of an int).
  • If you increment a double*, the address increases by 8 bytes.

A void* (void pointer) represents a raw memory address with unknown type and unknown size. Because the compiler does not know the size of void, it cannot calculate how many bytes to skip when you attempt to perform math on it.

This guide explains how to handle raw memory pointers correctly by casting them before performing arithmetic.

Understanding Pointer Arithmetic​

Pointer arithmetic in C# is scaled by the size of the type. NewAddress = OldAddress + (Index * sizeof(Type))

If Type is void, sizeof(void) is undefined (conceptually 0 or unknown). Therefore, the formula fails. To perform operations on a void pointer, you must first tell the compiler what size to use for the calculation.

Scenario: Attempting Math on void*​

This error frequently occurs when writing generic low-level memory manipulation code (like memcpy or buffer processing) where the function accepts a generic void*.

Example of error:

public unsafe class MemoryUtils
{
    public void AdvancePointer(void* ptr)
    {
        // ⛔️ Error CS0242: The operation in question is undefined on void pointers
        // The compiler asks: "Increment by how many bytes?" 
        // It doesn't know, because 'void' has no size.
        ptr++;
        
        // This is also invalid:
        // void* next = ptr + 1;
    }
}

Solution 1: Cast to a Byte Pointer (Most Common)​

If your goal is to move the pointer forward by a specific number of bytes, you should cast the void pointer to a byte*. Since sizeof(byte) is 1, incrementing a byte* moves the address exactly one byte forward. This is the standard way to perform raw address arithmetic.

Solution:

public unsafe class MemoryUtils
{
    public void AdvancePointer(void* ptr)
    {
        // 1. Cast to byte* (Size = 1 byte)
        byte* bytePtr = (byte*)ptr;

        // 2. Perform arithmetic
        // ✅ Correct: Moves the address forward by 1 byte.
        bytePtr++; 

        // If you need to assign it back to a void*, cast it back
        void* newPtr = (void*)bytePtr;
        
        System.Console.WriteLine($"Original: {(long)ptr}, New: {(long)newPtr}");
    }
}
note

Using byte* is the safest way to navigate raw memory when the structure of the data is unknown or irrelevant (e.g., copying data buffers).

Solution 2: Cast to the Specific Type​

If you know that the void* actually points to a specific structure (e.g., an array of integers), cast it to that specific pointer type. This ensures the arithmetic respects the size of the data structure (moving 4 bytes for integers, etc.).

Solution:

public unsafe void ProcessIntegers(void* rawData)
{
    // 1. Cast to int* (Size = 4 bytes)
    int* intPtr = (int*)rawData;

    // 2. Perform arithmetic
    // ✅ Correct: Moves forward by 4 bytes (to the next integer)
    intPtr++; 
    
    // OR access via index
    // ✅ Correct: Accesses the value at index 1 (4 bytes offset)
    int val = intPtr[1];
}

Conclusion​

CS0242 prevents mathematical ambiguity in memory operations.

  1. Identify the Type: If you know the data type (e.g., int), cast the void* to int*.
  2. Raw Bytes: If you just want to move the address by a specific byte offset, cast the void* to byte*.
  3. Never: Do not try to use ++, --, +, or - directly on a variable defined as void*.

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