c# – 如何使用动态LINQ进行求和

var myDataQueryable = _DataRawBase.AsQueryable<T>()
    .Sum("Price");

使用以下扩展方法：

public static object Sum(this IQueryable source,string member)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (member == null) throw new ArgumentNullException(nameof(member));

    // The most common variant of Queryable.Sum() expects a lambda.
    // Since we just have a string to a property,we need to create a
    // lambda from the string in order to pass it to the sum method.

    // Lets create a ((TSource s) => s.Price ). First up,the parameter "s":
    ParameterExpression parameter = Expression.Parameter(source.ElementType,"s");

    // Followed by accessing the Price property of "s" (s.Price):
    PropertyInfo property = source.ElementType.GetProperty(member);
    MemberExpression getter = Expression.MakeMemberAccess(parameter,property);

    // And finally,we create a lambda from that. First specifying on what
    // to execute when the lambda is called,and finally the parameters of the lambda.
    Expression selector = Expression.Lambda(getter,parameter);

    // There are a lot of Queryable.Sum() overloads with different
    // return types  (double,int,decimal,double?,int?,etc...).
    // We're going to find one that matches the type of our property.
    MethodInfo sumMethod = typeof(Queryable).GetMethods().First(
        m => m.Name == "Sum"
             && m.ReturnType == property.PropertyType
             && m.IsGenericMethod);

    // Now that we have the correct method,we need to know how to call the method.
    // Note that the Queryable.Sum<TSource>(source,selector) has a generic type,// which we haven't resolved yet. Good thing is that we can use copy the one from
    // our initial source expression.
    var genericSumMethod = sumMethod.MakeGenericMethod(new[] { source.ElementType });

    // TSource,source and selector are now all resolved. We now know how to call
    // the sum-method. We're not going to call it here,we just express how we're going
    // call it.
    var callExpression = Expression.Call(
        null,genericSumMethod,new[] {source.Expression,Expression.Quote(selector)});

    // Pass it down to the query provider. This can be a simple LinqToObject-datasource,// but also a more complex datasource (such as LinqToSql). Anyway,it knows what to
    // do.
    return source.Provider.Execute(callExpression);
}