Better error message for wrong assembly name in directives

src/Framework/Framework/Compilation/ControlTree/Resolved/DirectiveCompilationService.cs

@@ -6,6 +6,9 @@
 using DotVVM.Framework.Compilation.Binding;
 using System.Collections.Immutable;
 using DotVVM.Framework.Configuration;
+using System.Security.Cryptography;
+using System.Diagnostics;
+using FastExpressionCompiler;
 using DotVVM.Framework.Utils;
 
 namespace DotVVM.Framework.Compilation.ControlTree.Resolved
@@ -27,7 +30,8 @@ public DirectiveCompilationService(CompiledAssemblyCache compiledAssemblyCache,
         {
             if (CompileDirectiveExpression(directive, nameSyntax, imports) is not StaticClassIdentifierExpression expression)
             {
-                directive.AddError($"Could not resolve type '{nameSyntax.ToDisplayString()}'.");
+                var help = GetMissingTypeHelp(nameSyntax, imports);
+                directive.AddError($"Could not resolve type '{nameSyntax.ToDisplayString()}'.{(help is null ? "" : " " + help)}");
                 return null;
             }
             else return new ResolvedTypeDescriptor(expression.Type);
@@ -86,7 +90,38 @@ public DirectiveCompilationService(CompiledAssemblyCache compiledAssemblyCache,
             }
         }
 
-        private Expression? CompileDirectiveExpression(DothtmlDirectiveNode directive, BindingParserNode expressionSyntax, ImmutableList<NamespaceImport> imports)
+        private object? CreateDefaultValue(Type? type)
+        {
+            if (type != null && type.IsValueType)
+            {
+                return Activator.CreateInstance(type);
+            }
+            return null;
+        }
+
+        private string? GetMissingTypeHelp(BindingParserNode syntax, ImmutableList<NamespaceImport> imports)
+        {
+            try
+            {
+                // wrong assembly name:
+                if (syntax is AssemblyQualifiedNameBindingParserNode assemblyQualifiedName)
+                {
+                    var noAssemblyResolve = CompileDirectiveExpression(null, assemblyQualifiedName.TypeName, imports);
+                    if (noAssemblyResolve is StaticClassIdentifierExpression expression)
+                    {
+                        return $"Did you mean the '{expression.Type.Assembly.GetName().Name}' assembly? Note that assembly name is optional.";
+                    }
+                }
+                return null;
+            }
+            catch
+            {
+                Debug.Fail("This should not happen");
+                return null; // just fancy error handling, ignore errors
+            }
+        }
+
+        private Expression? CompileDirectiveExpression(DothtmlDirectiveNode? directive, BindingParserNode expressionSyntax, ImmutableList<NamespaceImport> imports)
         {
             TypeRegistry registry;
             if (expressionSyntax is TypeOrFunctionReferenceBindingParserNode typeOrFunction)
@@ -114,7 +149,7 @@ public DirectiveCompilationService(CompiledAssemblyCache compiledAssemblyCache,
             }
             catch (Exception ex)
             {
-                directive.AddError($"{expressionSyntax.ToDisplayString()} is not a valid type or namespace: {ex.Message}");
+                directive?.AddError($"{expressionSyntax.ToDisplayString()} is not a valid type or namespace: {ex.Message}");
                 return null;
             }
         }

src/Tests/Runtime/ControlTree/DefaultControlTreeResolver/ViewModelDirectiveTest.cs

@@ -112,5 +112,16 @@ public void ResolvedTree_ViewModel_DirectiveIdentifier_CaseInsensitivity(string
             Assert.IsFalse(root.Directives.Any(d => d.Value.Any(dd => dd.DothtmlNode.HasNodeErrors)));
             Assert.AreEqual(typeof(string), root.DataContextTypeStack.DataContextType);
         }
+
+        [TestMethod]
+        public void ResolvedTree_ViewModel_AssemblyNotFound_Error()
+        {
+            var root = ParseSource($"@viewModel DotVVM.Framework.Controls.IGridViewDataSet, DotVVM.Framework");
+
+            var directive = root.Directives["viewModel"].Single().DothtmlNode;
+            Assert.IsTrue(directive.HasNodeErrors);
+            StringAssert.Contains(directive.NodeErrors.Single(), "'DotVVM.Core'");
+        }
+
     }
 }