在字符串上构建表达式等于

Question

下面的代码适用于StartsWith,Contains但是当我尝试时,Equals我得到了这个异常:类型'System.String'上的多个方法'Equals'与提供的参数兼容.

要恢复,CreatePredicate可以构建3种类型的谓词,但只有Equals不起作用:

• AList.Where(x => x.MyField.StartsWith("ValueToSearch")); //OK
• AList.Where(x => x.MyField.Contains("ValueToSearch")); //OK
• AList.Where(x => x.MyField.Equals("ValueToSearch")); //Not OK

..

public enum TypeSearch
        {
            StartsWith = 1,
            Contains = 2,
            Equals = 3
        }

    private static Expression<Func<T, bool>> CreatePredicate<T>(string member, object value, TypeSearch type)
    {
        var p = System.Linq.Expressions.Expression.Parameter(typeof(T));
        System.Linq.Expressions.Expression body = p;
        foreach (var subMember in member.Split('.'))
        {
            body = System.Linq.Expressions.Expression.PropertyOrField(body, subMember);
        }

        var res = System.Linq.Expressions.Expression.Lambda<Func<T, bool>>(
                System.Linq.Expressions.Expression.Call(
                    System.Linq.Expressions.Expression.Call(
                        body,
                        "ToUpper", null),
                    type.ToString(), null,
                    System.Linq.Expressions.Expression.Constant(value.ToString().ToUpper())
                ), p);

        return res;
    }

Answer 1

正如错误消息所解释的那样,Equals在string类型上定义了多个方法- Equals(object)和Equals(string).由于您只使用名称来定位所需的方法,因此匹配不明确.

一种解决方案是MethodInfo将方法传递给Expression.Call例如

MethodInfo equalsMethod = typeof(string).GetMethod("Equals", new[] { typeof(string) });
Expression.Call(instanceExpr, equalsMethod, Expression.Constant(value.ToString().ToUpper()));