C# find class file path for any class

Here in 2023. I offer my code as another solution to this problem. You will require a few code analysis Dlls, but so far it works great and allowed me to open C# files directly from a right-click menu. I have something like 80-100 .cs files in my project and no SSD on the machine I'm using. Generally runs in about 1/4 of a second, but you could probably optimize if you want. I'm not sure what it does with generics, I haven't tested that.

 /// <summary>
 /// Utility class for code analysis.
 /// </summary>
 public static class CodeAnalysis
 {
     private static Dictionary<string, string> ClassNameToFileMapping = new Dictionary<string, string>();

     /// <summary>
     /// When this class is first loaded, we'll compile a list of all classes.
     /// </summary>
     static CodeAnalysis()
     {
         string[] cSharpFiles = Directory.GetFiles(Application.dataPath, "*.cs", SearchOption.AllDirectories);
         foreach (string cSharpFile in cSharpFiles)
         {
             SyntaxTree syntaxTree = CSharpSyntaxTree.ParseText(File.ReadAllText(cSharpFile));
             var root = syntaxTree.GetCompilationUnitRoot();
             foreach (SyntaxNode node in root.DescendantNodes())
             {
                 if (node is TypeDeclarationSyntax syntaxNode)
                 {
                     string fullname = syntaxNode.Identifier.ValueText;
                     IEnumerable<SyntaxNode> parentsAndSelf = syntaxNode.AncestorsAndSelf().Where(node => node is TypeDeclarationSyntax || node is NamespaceDeclarationSyntax);
                     StringBuilder stringBuilder = new StringBuilder();

                     bool typeEncountered = false;
                     bool anyEncountered = false;
                     foreach (SyntaxNode nameBuilderNode in parentsAndSelf.Reverse())
                     {
                         switch (nameBuilderNode)
                         {
                             case TypeDeclarationSyntax typeSyntax:
                                 if (typeEncountered) stringBuilder.Append("+");
                                 else if (anyEncountered) stringBuilder.Append(".");

                                 // Appending the type name
                                 stringBuilder.Append(typeSyntax.Identifier.ValueText);

                                 typeEncountered = true;
                                 break;
                             case NamespaceDeclarationSyntax namespaceSyntax:
                                 if (anyEncountered) stringBuilder.Append(".");

                                 // appending the namespace name
                                 stringBuilder.Append(namespaceSyntax.Name.GetText());

                                 break;
                             default:
                                 break;
                         }

                         anyEncountered = true;
                     }

                     // The carriage return can sometimes end up here. Need to remove it.
                     string className = stringBuilder.ToString().Replace("\r\n", "");

                     // We have to do this check for partial classes.
                     if (!ClassNameToFileMapping.ContainsKey(className))
                     {
                         ClassNameToFileMapping.Add(className, cSharpFile);
                     }
                 }
             }
         }
     }

     /// <summary>
     /// Get a file for a type.
     /// </summary>
     public static string GetFileForType(Type type)
     {
         return ClassNameToFileMapping[type.FullName];
     }

     public static string GetAssetPathForType(Type type)
     {
         string filePath = GetFileForType(type);
         string assetPath = "Assets" + filePath.Substring(Application.dataPath.Length);
         return assetPath;
     }
 }