* Migrate tests to xunit

* Update sample project to dotnetcore3.1
* Use Sqlite in sample project to run it everywhere
* Fix: Filter with escaped comma
* Fix: Filter "null" does not work with Contains or StartsWith
* Code cleanup: Adjust namespaces, adjust usings

SieveUnitTests/StringFilterNullTests.cs

@@ -0,0 +1,148 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Sieve.Models;
+using Sieve.Services;
+using SieveUnitTests.Entities;
+using SieveUnitTests.Services;
+using Xunit;
+
+namespace SieveUnitTests
+{
+    public class StringFilterNullTests
+    {
+        private readonly IQueryable<Comment> _comments;
+        private readonly SieveProcessor _processor;
+
+        public StringFilterNullTests()
+        {
+            _processor = new SieveProcessor(new SieveOptionsAccessor());
+
+            _comments = new List<Comment>
+            {
+                new Comment
+                {
+                    Id = 0,
+                    DateCreated = DateTimeOffset.UtcNow,
+                    Text = "This text contains null somewhere in the middle of a string",
+                    Author = "Dog",
+                },
+                new Comment
+                {
+                    Id = 1,
+                    DateCreated = DateTimeOffset.UtcNow, 
+                    Text = "null is here in the text",
+                    Author = "Cat",
+                },
+                new Comment
+                {
+                    Id = 2, 
+                    DateCreated = DateTimeOffset.UtcNow, 
+                    Text = "Regular comment without n*ll.",
+                    Author = "Mouse",
+                },
+                new Comment
+                {
+                    Id = 100, 
+                    DateCreated = DateTimeOffset.UtcNow, 
+                    Text = null,
+                    Author = "null",
+                },
+            }.AsQueryable();
+        }
+
+        [Fact]
+        public void Filter_Equals_Null()
+        {
+            var model = new SieveModel {Filters = "Text==null"};
+
+            var result = _processor.Apply(model, _comments);
+
+            Assert.Equal(100, result.Single().Id);
+        }
+
+        [Fact]
+        public void Filter_NotEquals_Null()
+        {
+            var model = new SieveModel {Filters = "Text!=null"};
+
+            var result = _processor.Apply(model, _comments);
+
+            Assert.Equal(new[] {0, 1, 2}, result.Select(p => p.Id));
+        }
+
+        [Theory]
+        [InlineData("Text@=null")]
+        [InlineData("Text@=*null")]
+        [InlineData("Text@=*NULL")]
+        [InlineData("Text@=*NulL")]
+        [InlineData("Text@=*null|text")]
+        public void Filter_Contains_NullString(string filter)
+        {
+            var model = new SieveModel {Filters = filter};
+
+            var result = _processor.Apply(model, _comments);
+
+            Assert.Equal(new[] {0, 1}, result.Select(p => p.Id));
+        }
+        
+        [Theory]
+        [InlineData("Text|Author==null", 100)]
+        [InlineData("Text|Author@=null", 0, 1, 100)]
+        [InlineData("Text|Author@=*null", 0, 1, 100)]
+        [InlineData("Text|Author_=null", 1, 100)]
+        [InlineData("Text|Author_=*null", 1, 100)]
+        public void MultiFilter_Contains_NullString(string filter, params int[] expectedIds)
+        {
+            var model = new SieveModel {Filters = filter};
+
+            var result = _processor.Apply(model, _comments);
+
+            Assert.Equal(expectedIds, result.Select(p => p.Id));
+        }
+
+        [Theory]
+        [InlineData("Text_=null")]
+        [InlineData("Text_=*null")]
+        [InlineData("Text_=*NULL")]
+        [InlineData("Text_=*NulL")]
+        [InlineData("Text_=*null|text")]
+        public void Filter_StartsWith_NullString(string filter)
+        {
+            var model = new SieveModel {Filters = filter};
+
+            var result = _processor.Apply(model, _comments);
+
+            Assert.Equal(new[] {1}, result.Select(p => p.Id));
+        }
+
+        [Theory]
+        [InlineData("Text!@=null")]
+        [InlineData("Text!@=*null")]
+        [InlineData("Text!@=*NULL")]
+        [InlineData("Text!@=*NulL")]
+        [InlineData("Text!@=*null|text")]
+        public void Filter_DoesNotContain_NullString(string filter)
+        {
+            var model = new SieveModel {Filters = filter};
+
+            var result = _processor.Apply(model, _comments);
+
+            Assert.Equal(new[] {2}, result.Select(p => p.Id));
+        }
+
+        [Theory]
+        [InlineData("Text!_=null")]
+        [InlineData("Text!_=*null")]
+        [InlineData("Text!_=*NULL")]
+        [InlineData("Text!_=*NulL")]
+        public void Filter_DoesNotStartsWith_NullString(string filter)
+        {
+            var model = new SieveModel {Filters = filter};
+
+            var result = _processor.Apply(model, _comments);
+
+            Assert.Equal(new[] {0, 2}, result.Select(p => p.Id));
+        }
+    }
+}