LINQ防爆pression转换/的毗连从int到字符串 [英] LINQ Expression Conversion / Concat from Int to string

问题描述

我想CONCAT为最后的前pression两位前pressions

 防爆pression＆LT;＆Func键LT; T，串＆GT;＆GT;
 

所以，我创建了前pression belwo code只有字符串类型工作得很好，如果我memberEx pression如的Int32或日期时间抛出异常

实施例$类型'System.Int32'的对$ pssion不能用于类型方法'System.String的毗连（System.String，System.String）'

的'System.String'的参数

如果我转换前pression为

  VAR转换=前pression.Convert（memberEx pression的typeof（字符串））;
 

没有得到强制经营者类型'System.Int32'和'System.String'之间。

请帮我解决。

code

  MethodInfo的bodyContactMethod = typeof运算（字符串）.GetMethod（的毗连，新的[] {typeof运算（字符串），typeof运算（字符串）}）;ParameterEx pression参数=前pression.Parameter（typeof运算（T））;机身前= pression.Call（bodyContactMethod，利弊，memberEx pression）;返回前pression.Lambda＆LT;＆Func键LT; T，串＆GT;＆GT;（机身，参数）;
 

解决方案

而不是试图转换为字符串，你可以尝试转换为对象，然后调用toString（）方法，就好像你在做：

  VAR转换= member.ToString（）;
 

作为Ex pression，它看起来是这样的：

  VAR convertedEx pression =前pression.Call（
                     防爆pression.Convert（memberEx pression的typeof（对象）），
                     typeof运算（对象）.GetMethod（的ToString））;
 

I want to Concat two expressions for the final expression

Expression<Func<T, string>>

So I have created expression belwo code works fine for only string types , If I get memberExpression as Int32 or DateTime throwing exception

Expression of type 'System.Int32' cannot be used for parameter of type 'System.String' of method 'System.String Concat(System.String, System.String)'

If I convert the expression as

var conversion = Expression.Convert(memberExpression, typeof (string));

getting No coercion operator is defined between types 'System.Int32' and 'System.String'.

Please help me to resolve

Code

MethodInfo bodyContactMethod = typeof (string).GetMethod("Concat",new[] {typeof (string), typeof (string)});

ParameterExpression parameter = Expression.Parameter(typeof (T));

body = Expression.Call(bodyContactMethod, cons, memberExpression);

return Expression.Lambda<Func<T, string>>(body, parameter);

解决方案

Instead of trying to cast to string, you could try casting to object then calling ToString(), as though you were doing:

var converted = member.ToString();

As an Expression, it will look something like this:

var convertedExpression = Expression.Call(
                     Expression.Convert(memberExpression, typeof(object)),
                     typeof(object).GetMethod("ToString"));

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