Why Do Identical ValueType.GetHashCode() Results Occur for Different String References Within a Struct?

Delving into the Eccentricity of ValueType.GetHashCode: A Value That May Not Vary

The intrigue surrounding how the native implementation of ValueType.GetHashCode() calculates hash codes for value types is undeniable. Let's unravel this enigma by examining a specific example and its unexpected outcome.

Consider two instances of the following structure:

struct TheKey
{
    public int id;
    public string Name;
}

Assigning distinct values to the Name field, we expect their hash codes to differ. However, the output reveals a startling reality:

var k1 = new TheKey(17, "abc");
var k2 = new TheKey(17, new string(new[] { 'a', 'b', 'c' }));

Console.WriteLine("hash1={0}", k1.GetHashCode());
Console.WriteLine("hash2={0}", k2.GetHashCode());

// Output:
// hash1=346948941
// hash2=346948941

Despite different string references, both k1 and k2 produce the same hash code.

Unveiling the Mechanics

The native implementation of ValueType.GetHashCode() operates through a surprisingly intricate mechanism. Primarily, it determines whether the structure contains reference type references or field gaps. If both conditions are absent, it performs an efficient bitwise XOR operation on all the value's bits, effectively combining all fields into the hash code. However, this approach is not universally applicable.

When reference types or gaps exist, the code embarks on a field-by-field traversal, searching for a usable field—either a value type or a non-null object reference. Once found, the hash code of this field, XOR'ed with the method table pointer, forms the final hash code.

The Mystery Unveiled

In our example, the usable field happens to be id. The string field, despite its varying values, is ignored, resulting in both k1 and k2 having the same id and, consequently, the same hash code.

Conclusion

Understanding this unconventional behavior reinforces the importance of carefully crafting value types for hash code computation. It is paramount to avoid relying solely on the CLR's default implementation. By explicitly defining hash code calculations, developers can ensure the uniqueness and consistency of hash codes for their value types.

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