如何在扩展方法中键入是否存在属性？

Question

我经常有这样的代码:

var stRecs = db.<someTable>
             .Where(a => a.DepID == depID)

选择单个记录,但是如果depID == 0我想取回所有记录.

我正在考虑创建一个扩展方法"WhereDepID_OrAll",就像

public static IQueryable<T> WhereDepID_OrAll<T>(this IQueryable<T> source)
        where T: // is what?
{
    if(depID > 0) { return source.Where(a => a.depID == depID); }
    else return source.Where(a => a);
}

现在我的基本问题是:我有几个表有depID - 我该如何设置Where T:？
该方法将如何确定表是否具有depID？

解决潜在问题的更好方法？

Answer 1

乍一看,反应将是:创建一个界面

public interface ObjectWithDepartmentInterface {
    int depID;
}

使用此depId实现此接口的所有实体,并使用

where T : ObjectWithDepartmentInterface

但linq to entities不接受查询中接口的属性...请参阅示例:基于接口生成的表达式

因此,唯一的方法是使用depId继承自具有depId属性的公共实体(可能是抽象)的实体.

并使用这个抽象实体作为

where T:

更简单(但更丑陋的方式)可能是不在T上添加约束,在方法中构建谓词,并在不良情况下抛出异常.

if (typeof(T).GetProperty("depId") == null)
   throw InvalidOperationException (string.Format("{0}" doesn't have a depId property, typeof(T).Name))

编辑

但也许这不是depId作为共同财产的问题然后

public static IQueryable<T> WhereExistsOrAll<T>(this IQueryable<T> source, string propertyName, int value)
        where T: // is what?
{
    if (value == 0)
    return source;
    var parameter = Expression.Parameter(typeof(T), "m");
    Expression member = parameter;
    member = Expression.Property(member, propertyName);
    member = Expression.Equals(member, Expression.Constant(value));
    var lambda = Expression.Lambda<Func<T, bool>>(member, new[]{parameter});
    return source.Where(lambda);
}

用法

var stRecs = db.<someTable>.WhereExistsOrAll("depId", depId);

编辑2

另一种方法是解析Predicate以获得"常量"值

类似的东西

public static IQueryable<T> GetAllOrRestrict<T>(this IQueryable<T> queryable, Expression<Func<T, bool>> predicate)
{
       var expression = predicate.Body as BinaryExpression;

       var rightPart = expression.Right as MemberExpression;
       var value = GetValue(rightPart);
       var test = value.ToString();
       int val;
       if (Int32.TryParse(value.ToString(), out val))
       {
           if (val != 0)
               return queryable.Where(predicate);
       }

       return queryable;
}

private static object GetValue(MemberExpression member)
{
     var objectMember = Expression.Convert(member, typeof(object));
     var getterLambda = Expression.Lambda<Func<object>>(objectMember);
     var getter = getterLambda.Compile();
     return getter();
}

用法

var stRecs = db.<someTable>.GetAllOrRestrict(m => m.depID == depId);