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);
