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How to Resolve Error "CS0637: The FieldOffset attribute is not allowed on static or const fields" in C#

The Compiler Error CS0637 is an attribute usage error related to memory layout. The message reads: "The FieldOffset attribute is not allowed on static or const fields".

In C#, the [FieldOffset] attribute is used to manually specify the position of a field within the memory of an instance of a struct or class.

  • Instance Fields: These define the shape and size of an object. Each new object has its own copy of these fields in memory.
  • Static/Const Fields: These belong to the Type itself. They are stored in a completely different area of memory (the High Frequency Heap or similar static storage), separate from any specific object instance.

Because static and const fields are not part of the object's instance memory layout, trying to assign them an offset relative to the "start of the object" is logically impossible.

Understanding Instance vs. Static Memory​

When you define a struct with [StructLayout(LayoutKind.Explicit)], you are defining the blueprint for the Instance.

[StructLayout(LayoutKind.Explicit)]
public struct Data
{
    [FieldOffset(0)] public int A; // Instance: Part of the struct's layout
}

If you add a const or static field, it exists outside this layout.

  • const: The value is baked into the code at compile time. It takes up 0 bytes in the struct instance.
  • static: The variable lives in a global static memory slot. It takes up 0 bytes in the struct instance.

Applying [FieldOffset(4)] to a static field is like trying to tell the compiler where to put the "Blueprints" inside the "House". They don't go there.

Scenario: Applying Offsets to Constants​

This error commonly happens when defining C-style structs that have "magic numbers" or configuration constants inside them.

Example of error:

using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit)]
public struct Packet
{
    // ⛔️ Error CS0637: The FieldOffset attribute is not allowed on static or const fields
    // This constant is just a definition. It is not part of the memory layout.
    [FieldOffset(0)]
    public const int HeaderSize = 10;

    [FieldOffset(0)]
    public byte StartByte;
}

Solution: Remove the Attribute​

Simply remove the [FieldOffset] attribute from any static or const member. Their position is managed by the runtime (for static) or the compiler (for const) and cannot be controlled manually.

Solution:

using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Explicit)]
public struct Packet
{
    // ✅ Correct: No offset needed. This value exists abstractly.
    public const int HeaderSize = 10;

    // ✅ Correct: Instance fields still need offsets.
    [FieldOffset(0)]
    public byte StartByte;
    
    [FieldOffset(1)]
    public int Payload;
}

Conclusion​

CS0637 is a logical check.

  1. Check Modifiers: Is the field static or const?
  2. Remove Offsets: If yes, delete [FieldOffset(...)]. It is unnecessary and invalid.
  3. Keep Layout: You only need offsets for standard instance fields (variables that change per object).

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