c# - LINQ 动态查询生成 'where AssignedUser is null'

Question

我正在尝试构建动态表达式查询以仅获取列中具有 null 的行

where AssignedUser is null

，下面是我的代码，但它没有按照我的预期进行。任何人都可以阐明这个问题吗？

private Expression<Func<VwAssignmentActivities, bool>> GetIsNullExpressionEquals<T>(string propName, T value)
{
    var item = Expression.Parameter(typeof(VwAssignmentActivities), "item");
    var prop = Expression.Convert(Expression.Property(item, propName), value.GetType());

    Expression body = Expression.Equal(prop, Expression.Constant(null, prop.Type));

    return Expression.Lambda<Func<VwAssignmentActivities, bool>>(body, item);
}

感谢任何帮助

Answer 1

您收到的错误是因为您的 Nullable 中有一个值。 get 类型返回 Int32，即使变量是 Nullable。那么您正在尝试将 null 转换为 int。

假设您只关心查找空值，我会做这样的事情

public Type GetNullable(Type type)
{
    if (type == typeof(Nullable<>))
        return  type.GetType();

    if(type == typeof(int))
        type = typeof(int?);
    else if(type == typeof(bool))
        type = typeof(bool?);
    else if(type == typeof(float))
        type = typeof(float?);      
    // etc. you  will have to build out every type you want.

    return type;
}

public Expression<Func<VwAssignmentActivities, bool>> GetIsNullExpressionEquals(string propName, Type value)
{       
    var item = Expression.Parameter(typeof(VwAssignmentActivities), "item");
    var prop = Expression.Convert(Expression.Property(item, propName), GetNullable(value));

    Expression body = Expression.Equal(prop, Expression.Constant(null, prop.Type));

    return Expression.Lambda<Func<VwAssignmentActivities, bool>>(body, item);
}