Why Do These Value Types Have the Same Hash Code Despite Different String Fields?

Understanding ValueType Hash Code Implementation

In the provided code, the two structures k1 and k2 have different string values in the Name field, yet surprisingly, they return the same hash code. This is because of the intricate implementation of GetHashCode for value types.

CLR's Handling of Value Types

For value types without reference fields or alignment gaps, the CLR employs a swift hashing mechanism by XOR-ing all bits in the structure. However, when there are reference fields or gaps, the CLR resorts to a more selective approach.

Selective Field Hashing

Instead of considering all fields, the CLR iterates through the structure's fields and identifies the first usable field (non-null value type or object reference). The hash code is then computed using that single field and the method table pointer, and the process concludes.

Field Selection

This explains why only the id field contributes to the hash code in the provided example, even though the Name field differs. Thus, swapping the order of id and Name fields ensures that the Name field is utilized for hashing.

Decimal Quirks

Notably, there's a bug in the CLR's fast hash calculation for structures containing Decimal values. The bits of a Decimal do not precisely reflect its numeric value, leading to incorrect hash code generation in certain cases.

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