C# 参数化Linq表达式帮助
我想做一个签名如下的方法:C# 参数化Linq表达式帮助,c#,linq,linq-to-entities,expression-trees,C#,Linq,Linq To Entities,Expression Trees,我想做一个签名如下的方法: Expression<Func<TSource, bool>> CreatePropertyFilter<TSource>(Expression<Func<TSource, string>> selector, string value, TextMatchMode matchMode); Expression<Func<string, bool>> comparerExpressi
Expression<Func<TSource, bool>> CreatePropertyFilter<TSource>(Expression<Func<TSource, string>> selector, string value, TextMatchMode matchMode);
Expression<Func<string, bool>> comparerExpression;
switch (matchMode)
{
case TextMatchMode.StartsWith:
comparerExpression = p => p.StartsWith(value);
break;
case TextMatchMode.EndsWith:
comparerExpression = p => p.EndsWith(value);
break;
case TextMatchMode.Contains:
comparerExpression = p => p.Contains(value);
break;
default:
comparerExpression = p => p.Equals(value);
break;
}
var equalityComparerParameter = Expression.Parameter(typeof(IncomingMail), null);
var equalityComparerExpression = Expression.Invoke(comparerExpression, Expression.Invoke(selector, equalityComparerParameter));
var equalityComparerPredicate = Expression.Lambda<Func<IncomingMail, bool>>(equalityComparerExpression, equalityComparerParameter);
问题是LINQ2实体不支持调用表达式
有什么建议吗
谢谢 基本上,给定一个选择器:
input => input.Member
您当前正在构造一个谓词表达式,如:
input => selector(input).Method(value)
相反,通过使用选择器表达式的主体(aMemberExpression
)来“展开”选择器表达式,以构造如下内容:
input => input.Member.Method(value)
这看起来像:
private static Expression<Func<TSource, bool>> CreatePropertyFilter<TSource>
(Expression<Func<TSource, string>> selector,
string value,
TextMatchMode matchMode)
{
// Argument-checking here.
var body = selector.Body as MemberExpression;
if (body == null)
throw new ArgumentException("Not a MemberExpression.");
// string.StartsWith / EndsWith etc. depending on the matchMode.
var method = typeof(string)
.GetMethod(GetMethodName(matchMode), new[] { typeof(string) });
// input.Member.method(value)
var compEx = Expression.Call(body, method, Expression.Constant(value));
// We can reuse the parameter of the source.
return Expression.Lambda<Func<TSource, bool>>(compEx, selector.Parameters);
}
谢谢你,安尼!这正是我要找的,工作起来很有魅力!我对lambda表达式的世界太新鲜了。
// I really don't like this enum.
// Why not explicitly include Equals as a member?
private static string GetMethodName(TextMatchMode mode)
{
switch (mode)
{
case TextMatchMode.StartsWith:
return "StartsWith";
case TextMatchMode.EndsWith:
return "EndsWith";
case TextMatchMode.Contains:
return "Contains";
default:
return "Equals";
}
}