EvaluationContext: Assure only the 3 types (decimal, string and bool)

VAR.ExpressionEvaluator/EvaluationContext.cs

@@ -15,11 +15,20 @@ namespace VAR.ExpressionEvaluator
 
         public void SetFunction(string name, Func<object[], object> function)
         {
+            if (_functions.ContainsKey(name))
+            {
+                _functions[name] = function;
+                return;
+            }
             _functions.Add(name, function);
         }
 
         public Func<object[], object> GetFunction(string name)
         {
+            if (_functions.ContainsKey(name) == false)
+            {
+                return null;
+            }
             return _functions[name];
         }
 
@@ -30,11 +39,29 @@ namespace VAR.ExpressionEvaluator
 
         public void SetVariable(string name, object value)
         {
+            if (value is DateTime)
+            {
+                value = ((DateTime)value).ToString("s");
+            }
+            if ((value is string) == false && (value is bool) == false)
+            {
+                value = Convert.ToDecimal(value);
+            }
+
+            if (_variables.ContainsKey(name))
+            {
+                _variables[name] = value;
+                return;
+            }
             _variables.Add(name, value);
         }
 
         public object GetVariable(string name)
         {
+            if (_variables.ContainsKey(name) == false)
+            {
+                return null;
+            }
             return _variables[name];
         }