How to implement HashCode.Combine using SymbolEqualityComparer

Question

After updating packages to .net7 in roslyn code generation project, has error RS1024 Use 'SymbolEqualityComparer' when comparing symbols for IEqualityComparer implementation of GetHashCode()

public int GetHashCode([DisallowNull] TypeSymbolMetaType? obj) => System.HashCode.Combine(obj.TypeSymbol);

How to fix HashCode.Combine using SymbolEqualityComparer

Welcome to SO. What is `TypeSymbolMetaType`? Can you show more of your code? — Tim Schmelter, Apr 26 '23 at 15:09
What the error message suggests would be using `public int GetHashCode([DisallowNull] TypeSymbolMetaType? obj) => SymbolEqualityComparer.Default.GetHashCode(obj.TypeSymbol);`, as [per docs](https://learn.microsoft.com/en-us/dotnet/api/microsoft.codeanalysis.symbolequalitycomparer.gethashcode?view=roslyn-dotnet) with some rationale explained [here](https://github.com/dotnet/roslyn-analyzers/issues/3427#issuecomment-604103434). — orhtej2, Apr 26 '23 at 15:16

score 0 · Accepted Answer · answered Apr 26 '23 at 15:32

Assuming TypeSymbol is ISymbol then you don't need to use Combine in this example:

public int GetHashCode([DisallowNull] TypeSymbolMetaType? obj) => SymbolEqualityComparer.Default.GetHashCode(obj.TypeSymbol);

In case you want to combine multiple properties the use the result of the SymbolEqualityComparer.Default.GetHashCode:

public int GetHashCode([DisallowNull] TypeSymbolMetaType? obj) => 
        System.HashCode.Combine(
            SymbolEqualityComparer.Default.GetHashCode(obj.TypeSymbol).
            ...);

P.S.

In some cases you might need to use SymbolEqualityComparer.IncludeNullability.

How to implement HashCode.Combine using SymbolEqualityComparer

1 Answers1