如何检查String类型的属性是否为null以使我的以下代码工作并且在方法调用期间不会失败？
- if (SelectedOperator is StringOperators)
- {
- MethodInfo method;
- var value = Expression.Constant(Value);
- switch ((StringOperators)SelectedOperator)
- {
- case StringOperators.Is:
- condition = Expression.Equal(property,value);
- break;
- case StringOperators.IsNot:
- condition = Expression.NotEqual(property,value);
- break;
- case StringOperators.StartsWith:
- method = typeof(string).GetMethod("StartsWith",new[] { typeof(string) });
- condition = Expression.Call(property,method,value);
- break;
- case StringOperators.Contains:
- method = typeof(string).GetMethod("Contains",value);
- break;
- case StringOperators.EndsWith:
- method = typeof(string).GetMethod("EndsWith",value);
- break;
- }
- }
解决方法
使用
AndAlso向结果表达式添加空检查,如下所示：
- // Your switch stays as is
- switch ((StringOperators)SelectedOperator) {
- case StringOperators.Is:
- condition = Expression.Equal(property,value);
- break;
- ...
- }
- // Create null checker property != null
- var nullCheck = Expression.NotEqual(property,Expression.Constant(null,typeof(object)));
- // Add null checker in front of the condition using &&
- condition = Expression.AndAlso(nullCheck,condition);
