How do Equals and GetHashCode work together in C#, and what changes when you use a record as a dictionary key?

Equality, GetHashCode, records, and dictionary keys

Connects equality semantics to hash-based collections and chooses identity or value equality deliberately.

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If two values are equal according to Equals, they must return the same hash code during the time they are used in a hash-based collection. Unequal values may share a hash code, so the collection still uses equality to resolve collisions. Classes use reference identity by default unless they override equality, while records generate value-based equality from their components. For a dictionary key I want equality to represent the key's real identity and every participating value to remain stable after insertion.

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Reference and value equality, equality contracts, hash stability, records, custom comparers, and dictionary correctness.

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Equality is not only about whether two comparisons return true. Hash-based collections depend on a stable contract, so a poor equality design can make an existing key appear to disappear.
If two values are equal according to Equals, they must return the same hash code during the time they are used in a hash-based collection. Unequal values may share a hash code, so the collection still uses equality to resolve collisions. Classes use reference identity by default unless they override equality, while records generate value-based equality from their components. For a dictionary key I want equality to represent the key's real identity and every participating value to remain stable after insertion.
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A record CustomerKey(Guid TenantId, string ExternalId) is a good key because its components describe identity and are immutable. A mutable record or class whose hash includes is dangerous if that property changes after insertion: the dictionary looks in a different bucket and may not find the same object. When equality rules are context-specific, such as case-insensitive external IDs, I would pass an instead of changing global equality for every use. I would also keep database identity and domain value equality conceptually separate.

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Overriding Equals without overriding GetHashCode consistently.
Using mutable properties in the hash of a dictionary key.
Persisting a runtime hash code as a durable external identifier.
Assuming records are suitable keys without checking which components define equality.

How do Equals and GetHashCode work together in C#, and what changes when you use a record as a dictionary key?

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