Add tests that assert that the new methods work

LiteDB.Tests/Issues/Pull2468_Tests.cs

@@ -0,0 +1,70 @@
+using FluentAssertions;
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+using Xunit;
+
+using static LiteDB.Tests.Issues.Issue1838_Tests;
+
+namespace LiteDB.Tests.Issues;
+
+public class Pull2468_Tests
+{
+    // tests if lowerinvariant works
+    [Fact]
+    public void Supports_LowerInvariant()
+    {
+        using var db = new LiteDatabase(":memory:");
+        var collection = db.GetCollection<TestType>(nameof(TestType));
+
+        collection.Insert(new TestType()
+        {
+            Foo = "Abc",
+            Timestamp = DateTimeOffset.UtcNow,
+        });
+
+        collection.Insert(new TestType()
+        {
+            Foo = "Def",
+            Timestamp = DateTimeOffset.UtcNow,
+        });
+
+        var result = collection.Query()
+            .Where(x => x.Foo.ToLowerInvariant() == "abc")
+            .ToList();
+
+        Assert.NotNull(result);
+        Assert.Single(result);
+    }
+
+    // tests if upperinvariant works
+    [Fact]
+    public void Supports_UpperInvariant()
+    {
+        using var db = new LiteDatabase(":memory:");
+        var collection = db.GetCollection<TestType>(nameof(TestType));
+
+        collection.Insert(new TestType()
+        {
+            Foo = "Abc",
+            Timestamp = DateTimeOffset.UtcNow,
+        });
+
+        collection.Insert(new TestType()
+        {
+            Foo = "Def",
+            Timestamp = DateTimeOffset.UtcNow,
+        });
+
+        var result = collection.Query()
+            .Where(x => x.Foo.ToUpperInvariant() == "ABC")
+            .ToList();
+
+        Assert.NotNull(result);
+        Assert.Single(result);
+    }
+}

LiteDB/Client/Mapper/Linq/TypeResolver/StringResolver.cs

@@ -5,6 +5,7 @@
 using System.Linq.Expressions;
 using System.Reflection;
 using System.Text;
+
 using static LiteDB.Constants;
 
 namespace LiteDB
@@ -59,4 +60,4 @@ public string ResolveMember(MemberInfo member)
 
         public string ResolveCtor(ConstructorInfo ctor) => null;
     }
-}
+}