feat(assertions): add CollectionNotNullAssertion for collection null checks with chaining support

TUnit.Assertions.Tests/NullabilityInferenceTests.cs

@@ -42,4 +42,30 @@ public async Task NotNull_ValueType_Chaining()
 
         Console.WriteLine(notNullValue.ToString());
     }
+
+    [Test]
+    public async Task NotNull_Dictionary_Chaining_With_Contains_Lambda()
+    {
+        // Regression test for issue #3471
+        // This should compile and work correctly
+        var dictionary = new Dictionary<string, string>
+        {
+            { "key", "value" }
+        };
+
+        // This works fine:
+        await Assert.That(dictionary).Contains(x => x.Key == "key");
+
+        // This should also work (was failing to compile before fix):
+        await Assert.That(dictionary).IsNotNull().And.Contains(x => x.Key == "key");
+    }
+
+    [Test]
+    public async Task NotNull_List_Chaining_With_Contains_Lambda()
+    {
+        var list = new List<int> { 1, 2, 3, 4, 5 };
+
+        // Verify that lambda type inference works after IsNotNull().And for lists
+        await Assert.That(list).IsNotNull().And.Contains(x => x > 3);
+    }
 }

TUnit.Assertions/Conditions/CollectionNullAssertion.cs

@@ -0,0 +1,32 @@
+using System.Collections;
+using TUnit.Assertions.Core;
+using TUnit.Assertions.Sources;
+
+namespace TUnit.Assertions.Conditions;
+
+/// <summary>
+/// Asserts that a collection is not null, preserving collection type information.
+/// Extends CollectionAssertionBase to ensure .And and .Or return collection-specific continuations.
+/// </summary>
+public class CollectionNotNullAssertion<TCollection, TItem> : CollectionAssertionBase<TCollection, TItem>
+    where TCollection : IEnumerable<TItem>
+{
+    public CollectionNotNullAssertion(AssertionContext<TCollection> context)
+        : base(context)
+    {
+    }
+
+    protected override Task<AssertionResult> CheckAsync(EvaluationMetadata<TCollection> metadata)
+    {
+        var value = metadata.Value;
+
+        if (value != null)
+        {
+            return Task.FromResult(AssertionResult.Passed);
+        }
+
+        return Task.FromResult(AssertionResult.Failed("value is null"));
+    }
+
+    protected override string GetExpectation() => "to not be null";
+}

TUnit.Assertions/Extensions/AssertionExtensions.cs

@@ -50,6 +50,22 @@ public static NotNullAssertion<TValue> IsNotNull<TValue>(
         return new NotNullAssertion<TValue>(mappedContext);
     }
 
+    /// <summary>
+    /// Asserts that a collection is not null, preserving collection type information.
+    /// Returns a collection-aware assertion that maintains TItem type for proper chaining.
+    /// This overload enables: Assert.That(collection).IsNotNull().And.Contains(x => predicate).
+    /// </summary>
+    public static CollectionNotNullAssertion<TCollection, TItem> IsNotNull<TCollection, TItem>(
+        this CollectionAssertionBase<TCollection, TItem> source)
+        where TCollection : class, IEnumerable<TItem>
+    {
+        var assertionSource = (IAssertionSource<TCollection>)source;
+        assertionSource.Context.ExpressionBuilder.Append(".IsNotNull()");
+        // Map from TCollection? to TCollection (nullable to non-nullable)
+        var mappedContext = assertionSource.Context.Map((TCollection? v) => v!);
+        return new CollectionNotNullAssertion<TCollection, TItem>(mappedContext);
+    }
+
     // ============ EQUALITY ============
     // IsEqualTo methods are now generated by AssertionExtensionGenerator for all assertion types
 

TUnit.PublicAPI/Tests.Assertions_Library_Has_No_API_Changes.DotNet10_0.verified.txt

@@ -507,6 +507,13 @@ namespace .Conditions
         protected override .<.> CheckAsync(.<TCollection> metadata) { }
         protected override string GetExpectation() { }
     }
+    public class CollectionNotNullAssertion<TCollection, TItem> : .<TCollection, TItem>
+        where TCollection : .<TItem>
+    {
+        public CollectionNotNullAssertion(.<TCollection> context) { }
+        protected override .<.> CheckAsync(.<TCollection> metadata) { }
+        protected override string GetExpectation() { }
+    }
     public abstract class ComparerBasedAssertion<TValue, TItem> : .<TValue>
     {
         protected ComparerBasedAssertion(.<TValue> context) { }
@@ -1850,6 +1857,8 @@ namespace .Extensions
             where TValue :  class { }
         public static .<TValue> IsNotNull<TValue>(this .<TValue?> source)
             where TValue :  struct { }
+        public static .<TCollection, TItem> IsNotNull<TCollection, TItem>(this .<TCollection, TItem> source)
+            where TCollection :  class, .<TItem> { }
         public static ..IsNotParsableIntoAssertion<T> IsNotParsableInto<[.(..None | ..PublicMethods | ..Interfaces)]  T>(this .<string> source) { }
         public static .<TValue> IsOfType<TValue>(this .<TValue> source,  expectedType, [.("expectedType")] string? expression = null) { }
         public static ..IsParsableIntoAssertion<T> IsParsableInto<[.(..None | ..PublicMethods | ..Interfaces)]  T>(this .<string> source) { }

TUnit.PublicAPI/Tests.Assertions_Library_Has_No_API_Changes.DotNet8_0.verified.txt

@@ -504,6 +504,13 @@ namespace .Conditions
         protected override .<.> CheckAsync(.<TCollection> metadata) { }
         protected override string GetExpectation() { }
     }
+    public class CollectionNotNullAssertion<TCollection, TItem> : .<TCollection, TItem>
+        where TCollection : .<TItem>
+    {
+        public CollectionNotNullAssertion(.<TCollection> context) { }
+        protected override .<.> CheckAsync(.<TCollection> metadata) { }
+        protected override string GetExpectation() { }
+    }
     public abstract class ComparerBasedAssertion<TValue, TItem> : .<TValue>
     {
         protected ComparerBasedAssertion(.<TValue> context) { }
@@ -1847,6 +1854,8 @@ namespace .Extensions
             where TValue :  class { }
         public static .<TValue> IsNotNull<TValue>(this .<TValue?> source)
             where TValue :  struct { }
+        public static .<TCollection, TItem> IsNotNull<TCollection, TItem>(this .<TCollection, TItem> source)
+            where TCollection :  class, .<TItem> { }
         public static ..IsNotParsableIntoAssertion<T> IsNotParsableInto<[.(..None | ..PublicMethods | ..Interfaces)]  T>(this .<string> source) { }
         public static .<TValue> IsOfType<TValue>(this .<TValue> source,  expectedType, [.("expectedType")] string? expression = null) { }
         public static ..IsParsableIntoAssertion<T> IsParsableInto<[.(..None | ..PublicMethods | ..Interfaces)]  T>(this .<string> source) { }

TUnit.PublicAPI/Tests.Assertions_Library_Has_No_API_Changes.DotNet9_0.verified.txt

@@ -507,6 +507,13 @@ namespace .Conditions
         protected override .<.> CheckAsync(.<TCollection> metadata) { }
         protected override string GetExpectation() { }
     }
+    public class CollectionNotNullAssertion<TCollection, TItem> : .<TCollection, TItem>
+        where TCollection : .<TItem>
+    {
+        public CollectionNotNullAssertion(.<TCollection> context) { }
+        protected override .<.> CheckAsync(.<TCollection> metadata) { }
+        protected override string GetExpectation() { }
+    }
     public abstract class ComparerBasedAssertion<TValue, TItem> : .<TValue>
     {
         protected ComparerBasedAssertion(.<TValue> context) { }
@@ -1850,6 +1857,8 @@ namespace .Extensions
             where TValue :  class { }
         public static .<TValue> IsNotNull<TValue>(this .<TValue?> source)
             where TValue :  struct { }
+        public static .<TCollection, TItem> IsNotNull<TCollection, TItem>(this .<TCollection, TItem> source)
+            where TCollection :  class, .<TItem> { }
         public static ..IsNotParsableIntoAssertion<T> IsNotParsableInto<[.(..None | ..PublicMethods | ..Interfaces)]  T>(this .<string> source) { }
         public static .<TValue> IsOfType<TValue>(this .<TValue> source,  expectedType, [.("expectedType")] string? expression = null) { }
         public static ..IsParsableIntoAssertion<T> IsParsableInto<[.(..None | ..PublicMethods | ..Interfaces)]  T>(this .<string> source) { }

TUnit.PublicAPI/Tests.Assertions_Library_Has_No_API_Changes.Net4_7.verified.txt

@@ -502,6 +502,13 @@ namespace .Conditions
         protected override .<.> CheckAsync(.<TCollection> metadata) { }
         protected override string GetExpectation() { }
     }
+    public class CollectionNotNullAssertion<TCollection, TItem> : .<TCollection, TItem>
+        where TCollection : .<TItem>
+    {
+        public CollectionNotNullAssertion(.<TCollection> context) { }
+        protected override .<.> CheckAsync(.<TCollection> metadata) { }
+        protected override string GetExpectation() { }
+    }
     public abstract class ComparerBasedAssertion<TValue, TItem> : .<TValue>
     {
         protected ComparerBasedAssertion(.<TValue> context) { }
@@ -1729,6 +1736,8 @@ namespace .Extensions
             where TValue :  class { }
         public static .<TValue> IsNotNull<TValue>(this .<TValue?> source)
             where TValue :  struct { }
+        public static .<TCollection, TItem> IsNotNull<TCollection, TItem>(this .<TCollection, TItem> source)
+            where TCollection :  class, .<TItem> { }
         public static ..IsNotParsableIntoAssertion<T> IsNotParsableInto<T>(this .<string> source) { }
         public static .<TValue> IsOfType<TValue>(this .<TValue> source,  expectedType, [.("expectedType")] string? expression = null) { }
         public static ..IsParsableIntoAssertion<T> IsParsableInto<T>(this .<string> source) { }