UO.TryGetProperty should determine the result value (#2592)

Issue https://github.com/microsoft/Power-Fx/issues/2591.

src/libraries/Microsoft.PowerFx.Core/Public/Values/UntypedObjectValue.cs

@@ -101,6 +101,31 @@ namespace Microsoft.PowerFx.Types
 
         public abstract bool TryGetProperty(string value, out IUntypedObject result);
 
+        /// <summary>
+        /// Get the property value of the untyped object.
+        /// Hosts should override this method in case a different return value is needed. For example, a host may want to return a ErrorValue in case of a missing property.
+        /// </summary>
+        /// <param name="value"></param>
+        /// <param name="returnType"></param>
+        /// <param name="span"></param>
+        /// <returns></returns>
+        public virtual FormulaValue GetProperty(string value, FormulaType returnType)
+        {
+            if (TryGetProperty(value, out var res))
+            {
+                if (res.Type == FormulaType.Blank)
+                {
+                    return new BlankValue(IRContext.NotInSource(returnType));
+                }
+
+                return new UntypedObjectValue(IRContext.NotInSource(returnType), res);
+            }
+            else
+            {
+                return new BlankValue(IRContext.NotInSource(returnType));
+            }
+        }
+
         public abstract bool TryGetPropertyNames(out IEnumerable<string> propertyNames);
 
         /// <summary>

src/libraries/Microsoft.PowerFx.Interpreter/EvalVisitor.cs

@@ -558,7 +558,12 @@ namespace Microsoft.PowerFx
             if (arg1 is UntypedObjectValue cov && arg2 is StringValue sv)
             {
                 if (cov.Impl.Type is ExternalType et && (et.Kind == ExternalTypeKind.Object || et.Kind == ExternalTypeKind.ArrayAndObject))
-                {
+                {
+                    if (cov.Impl is UntypedObjectBase untypedObjectBase)
+                    {
+                        return untypedObjectBase.GetProperty(sv.Value, node.IRContext.ResultType);
+                    }
+
                     if (cov.Impl.TryGetProperty(sv.Value, out var res))
                     {
                         if (res.Type == FormulaType.Blank)

src/tests/Microsoft.PowerFx.Interpreter.Tests.Shared/PadUntypedObjectTests.cs

@@ -6,6 +6,7 @@ using System.Collections.Generic;
 using System.Data;
 using System.Linq;
 using Microsoft.PowerFx.Core.IR;
+using Microsoft.PowerFx.Syntax;
 using Microsoft.PowerFx.Types;
 using Xunit;
 
@@ -138,6 +139,30 @@ namespace Microsoft.PowerFx.Interpreter.Tests
             Assert.Equal(97m, result.ToObject());
         }
 
+        [Fact]
+        public void PadUntypedObjectMissingProperty()
+        {
+            var uo1 = new PadUntypedObject(GetDataTable());
+            var uo2 = new PadUntypedObject2(GetDataTable());
+
+            var uov1 = new UntypedObjectValue(IRContext.NotInSource(FormulaType.UntypedObject), uo1);
+            var uov2 = new UntypedObjectValue(IRContext.NotInSource(FormulaType.UntypedObject), uo2);
+
+            RecalcEngine engine = new RecalcEngine();
+
+            engine.Config.SymbolTable.EnableMutationFunctions();
+            engine.UpdateVariable("padTable1", uov1);
+            engine.UpdateVariable("padTable2", uov2);
+
+            // PadUntypedObject does not override GetProperty. Returns blank if property is missing.
+            var result1 = engine.Eval(@"Index(padTable1, 1).Missing");
+            Assert.IsType<BlankValue>(result1);
+
+            // PadUntypedObject2 overrides GetProperty. Returns error if property is missing.
+            var result2 = engine.Eval(@"Index(padTable2, 1).Missing");
+            Assert.IsType<ErrorValue>(result2);
+        }
+
         private DataTable GetDataTable()
         {
             var dt = new DataTable("someTable");
@@ -562,6 +587,11 @@ namespace Microsoft.PowerFx.Interpreter.Tests
 
             throw new CustomFunctionErrorException("Something went wrong.", ErrorKind.InvalidArgument);
         }
+
+        public override FormulaValue GetProperty(string value, FormulaType returnType)
+        {
+            return new ErrorValue(IRContext.NotInSource(returnType), new ExpressionError() { Kind = ErrorKind.InvalidArgument });
+        }
     }
     #endregion
 }