具有泛型属性的泛型类上的 Linq

Question

问题是关于在一系列元素中获取最大值。 对象类型未知，属性遵循约定<EntityName>_Id。

目前我是这样解决的：

public static int GetMaxId<TEntity>()
{
    try
    {
        var key = typeof(TEntity).Name;
        var adapter = (IObjectContextAdapter)MyDbContext;
        var objectContext = adapter.ObjectContext;
        // 1. we need the container for the conceptual model
        var container = objectContext.MetadataWorkspace.GetEntityContainer(
            objectContext.DefaultContainerName, System.Data.Entity.Core.Metadata.Edm.DataSpace.CSpace);
        // 2. we need the name given to the element set in that conceptual model
        var name = container.BaseEntitySets.Where((s) => s.ElementType.Name.Equals(key)).FirstOrDefault().Name;
        // 3. finally, we can create a basic query for this set
        var query = objectContext.CreateQuery<TEntity>("[" + name + "]");

        // working with result
        var baseEntity = container.BaseEntitySets.Where((s) => s.ElementType.Name.Equals(key)).FirstOrDefault();
        var property = baseEntity.ElementType.Members.FirstOrDefault(_property => _property.Name.EndsWith("Id"));

        var tmpResult = query.ToList();

        var currentMaxID = tmpResult.Max(element =>
        {
            PropertyInfo pInfo = typeof(TEntity).GetProperty(property.Name);
            int value = (int)pInfo.GetValue(element);

            return value;
        });

        return currentMaxID;
    }
    catch (Exception ex)
    {
        string exMessage = "GetMaxId.Exception";
        EntityModelDataProviderLogger.Fatal(exMessage, ex);
        throw;
    }
}

有更好/更清洁的方法吗？

我读过关于动态表达式的文章，但对我来说不是很清楚。

LINQ expression with generic property 此示例比较日期。就我而言，我“只有一个元素”，我不知道如何构造表达式。

Answer 1

比较长，这里没法测试：

public static int GetMaxId<TEntity>()
{
    try
    {
        var key = typeof(TEntity).Name;
        var adapter = (IObjectContextAdapter)MyDbContext;
        var objectContext = adapter.ObjectContext;

        // 1. we need the container for the conceptual model
        var container = objectContext.MetadataWorkspace.GetEntityContainer(
            objectContext.DefaultContainerName, System.Data.Entity.Core.Metadata.Edm.DataSpace.CSpace);
        // 2. we need the name given to the element set in that conceptual model
        var baseEntity = container.BaseEntitySets.Single(s => s.ElementType.Name == key);
        // 3. finally, we can create a basic query for this set
        var query = objectContext.CreateQuery<TEntity>("[" + baseEntity.Name + "]");

        // Looking for the property
        string propertyId = baseEntity.Name + "_" + "Id";
        // The PropertyInfo connected to the EdmMember
        PropertyInfo property = (PropertyInfo)typeof(TEntity).GetProperty(propertyId);

        // Building the Expression
        ParameterExpression par = Expression.Parameter(typeof(TEntity));
        MemberExpression prop = Expression.Property(par, property);

        // cast to (int?)property
        // Note the use of int?, because the table could be empty!
        UnaryExpression conv = Expression.Convert(prop, typeof(int?));

        // An expression like x => x.Entity_Id
        Expression<Func<TEntity, int?>> lambda = Expression.Lambda<Func<TEntity, int?>>(conv, par);

        int? currentMaxID = ((IQueryable<TEntity>)query).Max(lambda);

        // We change null to 0
        return currentMaxID ?? 0;
    }
    catch (Exception ex)
    {
        string exMessage = "GetMaxId.Exception";
        EntityModelDataProviderLogger.Fatal(exMessage, ex);
        throw;
    }
}

如果它有效，我将解释它如何/为什么有效......

请注意，您所做的是概念上的错误...您使用ToList() 选择所有表，然后在本地查找Max()。此代码改为创建一个表达式树来执行 Max() 数据库端。

Answer 2

我将定义一个带有属性 Id 的接口，并在您的方法上添加一个约束，以仅接受该接口的类型。这消除了对脆弱的命名约定的需求，并且不需要反思。

public interface IEntity
{
    int Id { get; set; }
}

public static int GetMaxId<TEntity>() where TEntity : IEntity
{
    return MyDbContext.Set<TEntity>().AsEnumerable().Max(e => e.Id);
}

public Entity : IEntity
{
    public int MyId { get; set;}

    public int Id { get{ return MyId; } set{ MyId = value; } }
}

用法：

var max = GetMaxId<Entity>();