相关疑难解决方法(0)

我有一个扩展方法,使用字符串值动态过滤Linq到实体结果.它工作正常,直到我用它来过滤可空列.这是我的代码:

public static IOrderedQueryable<T> OrderingHelperWhere<T>(this IQueryable<T> source, string columnName, object value)
{
    ParameterExpression table = Expression.Parameter(typeof(T), "");
    Expression column = Expression.PropertyOrField(table, columnName);
    Expression where = Expression.GreaterThanOrEqual(column, Expression.Constant(value));
    Expression lambda = Expression.Lambda(where, new ParameterExpression[] { table });

    Type[] exprArgTypes = { source.ElementType };

    MethodCallExpression methodCall = Expression.Call(typeof(Queryable), 
                                                      "Where", 
                                                      exprArgTypes, 
                                                      source.Expression, 
                                                      lambda);

    return (IOrderedQueryable<T>)source.Provider.CreateQuery<T>(methodCall);
}

这是我如何使用它:

var results = (from row in ctx.MyTable select row)
              .OrderingHelperWhere("userId", 5);//userId is nullable column

这是我在将可用于可空表列时使用的异常:

没有为类型'System.Nullable`1 [System.Int32]'和'System.Int32'定义二元运算符GreaterThanOrEqual

我无法理解这一点.我该怎么办？

从5分钟前我提出的这个问题来看,很明显以下代码抛出异常,说明了这一点

未处理的异常:System.InvalidOperationException:没有为类型'System.Nullable`1 [System.Int32]'和'System.Int32'定义二进制运算符Equal.

码

public static void GetResultCollection<T>() {
        AccrualTrackingEntities db = new AccrualTrackingEntities();
        var result = db.CreateQuery<T>(String.Format("[{0}]", typeof(T).Name + "s"));

        int? ItemTypeValue = 1;

        var param = Expression.Parameter(typeof(T));

        var lambda = Expression.Lambda<Func<T, bool>>(
            Expression.Equal(
                Expression.Property(param, "ProcInstId"),
                Expression.Constant(ItemTypeValue)),
            param);

        var list = result.Where(lambda).ToList();
    }

但是,此代码具有明确列出的类型Expression.Constant确实有效

class Program {
    public static void GetResultCollection<T>() {
        AccrualTrackingEntities db = new AccrualTrackingEntities();
        var result = db.CreateQuery<T>(String.Format("[{0}]", typeof(T).Name + "s"));

        int? ItemTypeValue = 1;

        var param = Expression.Parameter(typeof(T));

        var lambda = …