using System;

using System.Linq;

using System.Collections.Generic;

using Microsoft.CodeAnalysis.CSharp;

using Microsoft.CodeAnalysis;

using Microsoft.CodeAnalysis.CSharp.Syntax;

​

var compilation = CSharpCompilation.Create("bla")

    .AddReferences(MetadataReference.CreateFromFile(typeof(string       ).Assembly.Location))

    .AddReferences(MetadataReference.CreateFromFile(typeof(Dictionary<,>).Assembly.Location))

    .AddSyntaxTrees(CSharpSyntaxTree.ParseText("class D<TKey>{public static System.Collections.Generic.Dictionary<TKey, string>.ValueCollection d;}"));

​

Console.WriteLine(

    new GenericContext(new(){ ["TKey"]=compilation.GetTypeByMetadataName(typeof(int).FullName) })

    .SubstituteGenericArgs(compilation.GetTypeByMetadataName("D`1").GetMembers().OfType<IFieldSymbol>().Single().Type)

);

​

public record GenericContext (Dictionary<string, ITypeSymbol> typeParameterValues)

{

    public ITypeSymbol SubstituteGenericArgs (ITypeSymbol type) => type switch

    {

        INamedTypeSymbol nts when nts.ContainingType is {} ct && ct.IsGenericType => SimpleSubstituteGenericArgs(

            SubstituteGenericArgs(ct).GetMembers().OfType<INamedTypeSymbol>().Single(

                gv => gv.OriginalDefinition == nts.OriginalDefinition

            )

        ),

        _ => SimpleSubstituteGenericArgs(type)

    };

​

    public ITypeSymbol SimpleSubstituteGenericArgs(ITypeSymbol type) => type switch

    {

        ITypeParameterSymbol tps => typeParameterValues[tps.Name],

        INamedTypeSymbol nts when nts.TypeArguments.Length == 0 => nts,

        INamedTypeSymbol nts when nts.TypeArguments.Length >  0 => nts.ConstructedFrom.Construct(

            nts.TypeArguments.Select(SubstituteGenericArgs).ToArray()

        )

    };

}