Add more descriptive error messages if a rule throws a NullReferenceE…

…xception (#2153)

Co-authored-by: Jeremy Skinner <90130+JeremySkinner@users.noreply.github.com>

Changelog.txt

@@ -1,5 +1,6 @@
-11.8.2 - 
+11.9.0 -
 Fix memory leak in NotEmptyValidator/EmptyValidator (#2174)
+Add more descriptive error messages if a rule throws a NullReferenceException (#2152)
 
 11.8.1 - 22 Nov 2023
 Fix unintentional behavioural changes in introduced in the previous release as part of #2158
@@ -34,7 +35,7 @@ Deprecated the ability to disable the root model null check via overriding Abstr
 Deprecated the Transform and TransformAsync methods (#2072)
 
 11.5.0 - 13 Feb 2023
-MemberNameValidatorSelector now supports wildcard indexes in property paths (#2056) 
+MemberNameValidatorSelector now supports wildcard indexes in property paths (#2056)
 Added overload of TestValidateAsync that accepts a context (#2052)
 Minor optimization to regex validator (#2035)
 Added Kazakh translations (#2036)

src/FluentValidation.Tests/NullTester.cs

@@ -18,6 +18,8 @@
 
 namespace FluentValidation.Tests;
 
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using Xunit;
 
@@ -60,4 +62,31 @@ public class NullTester {
 		var result = validator.Validate(new Person());
 		result.IsValid.ShouldBeTrue();
 	}
-}
+
+	[Fact]
+	public void NullProperty_should_throw_NullReferenceException() {
+		var validator = new InlineValidator<Person>();
+		validator.RuleFor(x => x.Orders.Count).NotEmpty();
+
+		var ex = Assert.Throws<NullReferenceException>(() => validator.Validate(new Person {
+			Orders = null
+		}));
+
+		ex.Message.ShouldEqual("NullReferenceException occurred when executing rule for x => x.Orders.Count. If this property can be null you should add a null check using a When condition");
+		ex.InnerException.ShouldNotBeNull();
+		ex.InnerException!.GetType().ShouldEqual(typeof(NullReferenceException));
+	}
+
+	[Fact]
+	public void ForEachNullProperty_should_throw_NullReferenceException_when_exception_occurs() {
+		var validator = new InlineValidator<Person>();
+		validator.RuleForEach(x => x.Orders[0].Payments).NotNull();
+
+		var ex = Assert.Throws<NullReferenceException>(() => validator.Validate(new Person {
+			Orders = null
+		}));
+		ex.Message.ShouldEqual("NullReferenceException occurred when executing rule for x => x.Orders.get_Item(0).Payments. If this property can be null you should add a null check using a When condition");
+		ex.InnerException.ShouldNotBeNull();
+		ex.InnerException!.GetType().ShouldEqual(typeof(NullReferenceException));
+	}
+}

src/FluentValidation.Tests/Person.cs

@@ -46,6 +46,8 @@ public class Person {
 	public Address Address { get; set; }
 	public IList<Order> Orders { get; set; }
 
+	public Order LatestOrder { get; set; }
+
 	public string Email { get; set; }
 	public decimal Discount { get; set; }
 

src/FluentValidation/Internal/CollectionPropertyRule.cs

@@ -134,7 +134,14 @@ public CollectionPropertyRule(MemberInfo member, Func<T, IEnumerable<TElement>>
 		}
 
 		var cascade = CascadeMode;
-		var collection = PropertyFunc(context.InstanceToValidate) as IEnumerable<TElement>;
+		IEnumerable<TElement> collection;
+
+		try {
+			collection = PropertyFunc(context.InstanceToValidate);
+		}
+		catch (NullReferenceException nre) {
+			throw new NullReferenceException($"NullReferenceException occurred when executing rule for {Expression}. If this property can be null you should add a null check using a When condition", nre);
+		}
 
 		int count = 0;
 		int totalFailures = context.Failures.Count;

src/FluentValidation/Internal/PropertyRule.cs

@@ -145,7 +145,12 @@ TProperty PropertyFunc(T instance)
 
 			if (first) {
 				first = false;
-				propValue = PropertyFunc(context.InstanceToValidate);
+				try {
+					propValue = PropertyFunc(context.InstanceToValidate);
+				}
+				catch (NullReferenceException nre) {
+					throw new NullReferenceException($"NullReferenceException occurred when executing rule for {Expression}. If this property can be null you should add a null check using a When condition", nre);
+				}
 			}
 
 			bool valid = await component.ValidateAsync(context, propValue, useAsync, cancellation);