[英]Null check for lambda expression tree
How can I check if property of String type is null in order my following code to work and not fail during method calling ? 如何检查String类型的属性是否为null以便我的以下代码正常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", new[] { typeof(string) });
condition = Expression.Call(property, method, value);
break;
case StringOperators.EndsWith:
method = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
condition = Expression.Call(property, method, value);
break;
}
}
Add a null check to the resultant expression using AndAlso
, like this: 使用
AndAlso
将空检查添加到结果表达式中,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);
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