Couple bug fixes - exists with case sensitivity and primitive != operator

src/Gridify/Builder/BaseQueryBuilder.cs

@@ -217,8 +217,13 @@ private static object AddIndexerNullCheck(LambdaExpression mapTarget, object que
                             && op.Kind is not SyntaxKind.GreaterOrEqualThan
                             && op.Kind is not SyntaxKind.LessOrEqualThan)
       {
-         value = value.ToString()?.ToLower();
-         body = Expression.Call(body, MethodInfoHelper.GetToLowerMethod());
+         var strLowerValue = value.ToString()?.ToLower();
+         value = strLowerValue;
+
+         if(!string.IsNullOrEmpty(strLowerValue))
+         {
+            body = Expression.Call(body, MethodInfoHelper.GetToLowerMethod());   
+         }
       }
 
       var query = BuildQueryAccordingToValueType(body, parameter, value, op, valueExpression);

src/Gridify/Builder/LinqQueryBuilder.cs

@@ -7,7 +7,7 @@
 
 namespace Gridify.Builder;
 
 public class LinqQueryBuilder<T>(IGridifyMapper<T> mapper) : BaseQueryBuilder<Expression<Func<T, bool>>, T>(mapper)
 {
    protected override Expression<Func<T, bool>>? BuildNestedQuery(
       Expression body, IGMap<T> gMap, ValueExpressionSyntax value, ISyntaxNode op)
@@ -364,6 +364,9 @@
       if (op.Kind == SyntaxKind.NotEqual)
       {
          containsExp = Expression.Not(containsExp);
+         // issue #204 we need to add or null check as well for Not Contains
+         containsExp = Expression.OrElse(Expression.Equal(prop, Expression.Constant(null)), containsExp);
+         return Expression.Lambda(containsExp, param);
       }
       return GetExpressionWithNullCheck(prop, param, containsExp);
    }

test/Gridify.Tests/GridifyExtensionsShould.cs

@@ -52,6 +52,7 @@ public static IEnumerable<TestClass> GetSampleData()
       lst.Add(new TestClass(27, "ali reza", null));
       lst.Add(new TestClass(27, "[ali]", null));
       lst.Add(new TestClass(28, @"Esc/\pe", null));
+      lst.Add(new TestClass(29, @"NullTag", null, tag: null));
 
       return lst;
    }

test/Gridify.Tests/IssueTests/Issue202Tests.cs

@@ -0,0 +1,48 @@
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace Gridify.Tests.IssueTests;
+
+public class Issue202Tests
+{
+    private readonly List<TestClass> _fakeRepository = [.. GridifyExtensionsShould.GetSampleData()];
+
+    [Fact]
+    // Issue #202
+    public void ApplyFiltering_NotExists_GlobalCaseInsensitiveSearch()
+    {
+        var mapper = new GridifyMapper<TestClass>(m => m.CaseInsensitiveFiltering = true).GenerateMappings();
+
+        var gq = new GridifyQuery { Filter = "tag=" };
+
+        var expected = _fakeRepository.Where(q => string.IsNullOrEmpty(q.Tag)).ToList();
+
+        var actual = _fakeRepository.AsQueryable()
+            .ApplyFiltering(gq, mapper)
+            .ToList();
+
+        Assert.Equal(expected.Count, actual.Count);
+        Assert.Equal(expected, actual);
+        Assert.True(actual.Any());
+    }
+
+    [Fact]
+    // Issue #202
+    public void ApplyFiltering_Exists_GlobalCaseInsensitiveSearch()
+    {
+        var mapper = new GridifyMapper<TestClass>(m => m.CaseInsensitiveFiltering = true).GenerateMappings();
+
+        var gq = new GridifyQuery { Filter = "tag!=" };
+
+        var expected = _fakeRepository.Where(q => !string.IsNullOrEmpty(q.Tag)).ToList();
+
+        var actual = _fakeRepository.AsQueryable()
+            .ApplyFiltering(gq, mapper)
+            .ToList();
+
+        Assert.Equal(expected.Count, actual.Count);
+        Assert.Equal(expected, actual);
+        Assert.True(actual.Any());
+    }
+}

test/Gridify.Tests/IssueTests/Issue204Tests.cs

@@ -0,0 +1,33 @@
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace Gridify.Tests.IssueTests;
+
+public class Issue204Tests
+{
+    [Fact]
+    public void ApplyFiltering_NotEquals_ShouldMatch_NullItems()
+    {
+        // arrange
+        var dataSource = new List<Test>()
+        {
+            new() {FavouriteColorList = ["Green", "Blue"]},
+            new() {FavouriteColorList = ["White", "Yellow"]},
+            new() { FavouriteColorList = null },
+        }.AsQueryable();
+
+        var expected = dataSource.Where(q => q.FavouriteColorList == null || !q.FavouriteColorList.Contains("Green")).ToList();
+        var actual = dataSource.ApplyFiltering("FavouriteColorList!=Green").ToList();
+
+        // assert
+        Assert.Equal(expected.Count, actual.Count);
+        Assert.Equal(expected, actual);
+        Assert.True(actual.Any());
+    }
+
+    private class Test
+    {
+        public List<string> FavouriteColorList { get; set; }
+    }
+}