Support indexers in expression trees when generating PropertyChain fo…

…r simple cases.

src/FluentValidation.Tests/PropertyChainTests.cs

@@ -83,6 +83,13 @@ public class PropertyChainTests {
 		chain.ToString().ShouldEqual("Address.Id");
 	}
 
+	[Fact]
+	public void Creates_from_expression_with_indexer() {
+		Expression<Func<Person, string>> expr = x => x.Orders[0].ProductName;
+		var chain = PropertyChain.FromExpression(expr);
+		chain.ToString().ShouldEqual("Orders[0].ProductName");
+	}
+
 	[Fact]
 	public void Should_ignore_blanks() {
 		chain.Add("");

src/FluentValidation/Internal/PropertyChain.cs

@@ -70,10 +70,29 @@ public class PropertyChain {
 		});
 
 		var memberExp = getMemberExp(expression.Body);
+		string indexer = null;
 
 		while(memberExp != null) {
-			memberNames.Push(memberExp.Member.Name);
-			memberExp = getMemberExp(memberExp.Expression);
+			string propertyName = memberExp.Member.Name;
+
+			if (indexer != null) {
+				propertyName += "[" + indexer + "]";
+				indexer = null;
+			}
+
+			memberNames.Push(propertyName);
+
+			// Handle indexers.
+			if (memberExp.Expression is MethodCallExpression mce) {
+				if (mce.Method is {IsSpecialName: true, Name: "get_Item"} && mce.Arguments.Count == 1 && mce.Arguments[0] is ConstantExpression ce) {
+					indexer = ce.Value?.ToString();
+					memberExp = getMemberExp(mce.Object);
+				}
+			}
+			else {
+				memberExp = getMemberExp(memberExp.Expression);
+			}
+
 		}
 
 		return new PropertyChain(memberNames);