表达对创建的泛型类型参数

我试图建立一个表达式与泛型类型参数个数可变创建通用Tuple<>实例变量数量的元组。表达对创建的泛型类型参数

生成的Tuple<>实例的想法是基于其上具有KeyAttribute的属性为实体类型动态创建组合键值。然后组合键将被用作Dictionary<object, TEntity>中的键。所以lambda表达式应该建立一定的实体类型和拉姆达会被调用，传递的TEntity一个实例来取回一个Tuple<>形式的组合键。

例实体模型

public class MyEntityModel 
{ 
    [Key] 
    public string Key1 { get; set; } 
    [Key] 
    public Guid Key2 { get; set; } 
    public int OtherProperty { get; set; } 
}

表达什么应该做

public Func<MyEntityModel, object> BuildKeyFactory() 
{ 
    // This is how the LambdaExpression should look like, but then for a generic entity type instead of fixed to MyEntityModel 
    return new Func<MyEntityModel, object>(entity => new Tuple<string, Guid>(entity.Key1, entity.Key2)); 
}

但当然的实体模型需要一个泛型类型。

我有什么到目前为止

public Func<TEntity, object> BuildKeyFactory<TEntity>() 
{ 
    var entityType = typeof(TEntity); 

    // Get properties that have the [Key] attribute 
    var keyProperties = entityType.GetProperties(BindingFlags.Instance | BindingFlags.Public) 
     .Where(x => x.GetCustomAttribute(typeof(KeyAttribute)) != null) 
     .ToArray(); 

    var tupleType = Type.GetType($"System.Tuple`{keyProperties.Length}"); 
    if (tupleType == null) throw new InvalidOperationException($"No tuple type found for {keyProperties.Length} generic arguments"); 

    var keyPropertyTypes = keyProperties.Select(x => x.PropertyType).ToArray(); 
    var tupleConstructor = tupleType.MakeGenericType(keyPropertyTypes).GetConstructor(keyPropertyTypes); 
    if (tupleConstructor == null) throw new InvalidOperationException($"No tuple constructor found for key in {entityType.Name} entity"); 

    // The following part is where I need some help with... 
    var newTupleExpression = Expression.New(tupleConstructor, keyProperties.Select(x => ????)); 

    return Expression.Lambda<Func<TEntity, object>>(????).Compile(); 
}

正如你所看到的，我无法弄清楚如何我需要为传递给Expression.New()调用（可能是一些与Expression.MakeMemberAccess(Expression.Property())创建属性表达式，但不知道如何从lambda参数中传递TEntity实例），以及如何通过Expression.Lambda调用将其链接起来。任何帮助将不胜感激！

来源

2017-11-11 Wouter Huysentruit

你是相当接近。

// we need to build entity => new Tuple<..>(entity.Property1, entity.Property2...) 
// arg represents "entity" above 
var arg = Expression.Parameter(typeof(TEntity)); 
// The following part is where I need some help with... 
// Expression.Property(arg, "name) represents "entity.Property1" above 
var newTupleExpression = Expression.New(tupleConstructor, keyProperties.Select(c => Expression.Property(arg, c))); 
return Expression.Lambda<Func<TEntity, object>>(newTupleExpression, arg).Compile();

来源

2017-11-11 10:28:42 Evk

表达对创建的泛型类型参数

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