Returning a nested generic Expression>

问题:

The error message is "The type or namespace name 'T' could not be found."

???

public static Expression<Func<T, bool>> MakeFilter(string prop, object val)
{
    ParameterExpression pe = Expression.Parameter(typeof(T), "p");
    PropertyInfo pi = typeof(T).GetProperty(prop);
    MemberExpression me = Expression.MakeMemberAccess(pe, pi);
    ConstantExpression ce = Expression.Constant(val);
    BinaryExpression be = Expression.Equal(me, ce);
    return Expression.Lambda<Func<T, bool>>(be, pe);
}

Related links:

Using reflection to address a Linqed property

http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/df9dba6e-4615-478d-9d8a-9fd80c941ea2/

Runtime creation of generic Func<T>

回答1:

You need to make the method itself generic:

public static Expression<Func<T, bool>> MakeFilter<T>(string prop, object val)
                                                  -+-
                                                   ^
                                                   +- this

回答2:

There's no generic argument defined for your method. You should define one (MakeFilter<T>):

public static Expression<Func<T, bool>> MakeFilter<T>(string prop, object val)
{
    ParameterExpression pe = Expression.Parameter(typeof(T), "p");
    PropertyInfo pi = typeof(T).GetProperty(prop);
    MemberExpression me = Expression.MakeMemberAccess(pe, pi);
    ConstantExpression ce = Expression.Constant(val);
    BinaryExpression be = Expression.Equal(me, ce);
    return Expression.Lambda<Func<T, bool>>(be, pe);
}

回答3:

The method needs to be declared as generic (MakeFilter<T>):

public static Expression<Func<T, bool>> MakeFilter<T>(string prop, object val)

Otherwise, how else would the caller be able to specify what T is?