Fix ExpressionEqualsNode not comparing null and string

VAR.ExpressionEvaluator.Tests/ParserTests.cs

@@ -495,6 +495,46 @@ namespace VAR.ExpressionEvaluator.Tests
             Assert.False((bool?)result);
         }
 
+        [Fact()]
+        public void Relations__Var1NullEquals666_EqualsFalse()
+        {
+            EvaluationContext evaluationContex = new EvaluationContext();
+            evaluationContex.SetVariable("v1", null);
+            string expression = "v1 = 666";
+            object result = Parser.EvaluateString(expression, evaluationContex);
+            Assert.False((bool?)result);
+        }
+
+        [Fact()]
+        public void Relations__Var1NullEqualsStringHello_EqualsFalse()
+        {
+            EvaluationContext evaluationContex = new EvaluationContext();
+            evaluationContex.SetVariable("v1", null);
+            string expression = "v1 = \"Hello\"";
+            object result = Parser.EvaluateString(expression, evaluationContex);
+            Assert.False((bool?)result);
+        }
+
+        [Fact()]
+        public void Relations__Var1666Equals666_EqualsFalse()
+        {
+            EvaluationContext evaluationContex = new EvaluationContext();
+            evaluationContex.SetVariable("v1", 666);
+            string expression = "v1 = 666";
+            object result = Parser.EvaluateString(expression, evaluationContex);
+            Assert.True((bool?)result);
+        }
+
+        [Fact()]
+        public void Relations__Var1HelloEqualsStringHello_EqualsFalse()
+        {
+            EvaluationContext evaluationContex = new EvaluationContext();
+            evaluationContex.SetVariable("v1", "Hello");
+            string expression = "v1 = \"Hello\"";
+            object result = Parser.EvaluateString(expression, evaluationContex);
+            Assert.True((bool?)result);
+        }
+
         #endregion Relations
 
         #region BooleanOps

VAR.ExpressionEvaluator/EvaluationContext.cs

@@ -48,7 +48,7 @@ namespace VAR.ExpressionEvaluator
             {
                 value = ((DateTime)value).ToString("s");
             }
-            if ((value is string) == false && (value is bool) == false)
+            if ((value is string) == false && (value is bool) == false && value != null)
             {
                 value = Convert.ToDecimal(value);
             }

VAR.ExpressionEvaluator/ExpressionNodes/ExpressionEqualsNode.cs

@@ -16,6 +16,11 @@ namespace VAR.ExpressionEvaluator
                 return string.Compare((string)leftValue, (string)rightValue) == 0;
             }
 
+            if ((leftValue is string && rightValue == null) || (leftValue == null && rightValue is string))
+            {
+                return false;
+            }
+
             if (leftValue is string)
             {
                 if (string.IsNullOrEmpty((string)leftValue))