我经常有这样的代码：
var stRecs = db.<someTable>
.Where(a => a.DepID == depID)
选择一条记录，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”，比如
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：我有几个带有 depID 的表 - 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？
乍一看，反应是：创建一个接口
public interface ObjectWithDepartmentInterface {
int depID;
}
使所有使用此 depId 的实体实现此接口，并使用
where T : ObjectWithDepartmentInterface
但是 linq to entity 不接受查询中接口的属性...参见例如：基于接口生成的表达式
因此，唯一的方法是使您的实体具有从具有 depId 属性的公共实体（可能是抽象的）继承的 depId。
并使用这个抽象实体作为
where T:
一种更简单（但更丑陋的方法）可能是不对 T 添加约束，在方法中构建谓词，并在坏情况下抛出异常。
if (typeof(T).GetProperty("depId") == null)
throw InvalidOperationException (string.Format("{0}" doesn't have a depId property, typeof(T).Name))
编辑
但也许不是 depId 作为公共属性的问题 那么
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
另一种方法是解析谓词以获得“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);
I know it's not particularly fashionable, but isn't this exactly what the Query Builder methods in Entity Framework are for?
var stRecs = db.<someTable>
.Where("it.DepID == @depID OR @depID = 0",
new ObjectParameter("depID", depID));
This works on any someTable such that it has a column named DepID. It can of course be made an extension method:
public static ObjectQuery<T> WhereIdEqualOrAll<T>(this ObjectQuery<T> q, int depID)
where T : class
{
return q.Where("it.DepID = @id OR @id = 0", new ObjectParameter("id", id));
}
to be invoked thus:
var stRecs = db.<someTable>.WhereIdEqualOrAll(depID);
使用接口：
public interface IInterface
{
int depId;
}
这将强制T继承IInterface和实施depId.
然后你可以将它添加到扩展中：
public static IQueryable<T> WhereDepID_OrAll<T>(this IQueryable<T> source) where T: IInterface
{
}
Raphaels Edit 2 完成了这项工作！
小型版：如果存在 DepID，我如何包含 NULL 值？
我喜欢返回 ID == x OR ID == NULL 的所有部门
也许还有一个额外的 bool includeNullValues)
public static IQueryable<T> GetAllOrRestrict<T>(this IQueryable<T> queryable,
Expression<Func<T, bool>> predicate,
bool includeNullValues)
{
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)
{
if (includeNullValues)
{
var parameter = Expression.Parameter(typeof(T),"m");
return queryable.Where(predicate) <====HOW to " || depID == null) ???
}
else
{
return queryable.Where(predicate);
}
}
}
return queryable;
}
使用接口，然后手动构建表达式树，引用实际的类。