当我有一个表达式时，如何获得 Expression<System.Func<TEntity, TEntity>>？

Question

我正在研究服务的通用 PATCH 方法。

public virtual void Patch ( int id, Delta<TEntityView> view )
{
    var type = typeof( TEntity );
    TEntity model = Activator.CreateInstance( type ) as TEntity;

    foreach( var changedProperty in view.GetChangedPropertyNames() )
    {
        var property = type.GetProperty( changedProperty );
        var propertyType = property.PropertyType;
        var retreived = view.TryGetPropertyValue( changedProperty, out object propval );

        if ( retreived && property != null )
        {
            property.SetValue( model, propval, null );
        }
    }

    UnitOfWork.Query<TEntity>( e => e.Id == id )
        .Update( m => model );
}

在 Update 声明中，我得到

System.Exception: 'Invalid Cast. The update expression must be of type MemberInitExpression.'

该方法定义在Entity Framework Plus (https://entityframework-plus.net/):

#region Assembly Z.EntityFramework.Plus.EF6, Version=1.12.14.0, Culture=neutral, PublicKeyToken=59b66d028979105b
public static int Update<T>(this IQueryable<T> query, Expression<Func<T, T>> updateFactory) where T : class;

我改变了我的方法如下：

public virtual void Patch ( int id, Delta<TEntityView> view )
{
    var type = typeof( TEntity );
    var bindings = new List<MemberBinding>();

    foreach( var changedProperty in view.GetChangedPropertyNames() )
    {
        var property = type.GetProperty( changedProperty );
        var propertyType = property.PropertyType;
        var retreived = view.TryGetPropertyValue( changedProperty, out object propval );

        if ( retreived && property != null )
        {
            bindings.Add( Expression.Bind( type.GetMember( changedProperty )[0], Expression.Constant( propval ) ) );
        }
    }

    Expression ex = Expression.MemberInit( Expression.New( type ), bindings );

    // Expression <-- have this
    // Expression<Func<TModel, TModel>> updateFactory <-- need this

    UnitOfWork.Query<TEntity>( e => e.Id == id )
        .Update( ex );
}

现在，在更新中，我收到一条红色波浪线消息： Argument 1: cannot convert from 'System.Linq.Expressions.Expression' to 'System.Linq.Expressions.Expression<System.Func<TEntity, TEntity>>'

我确信我遗漏了一些小东西来让魔法发挥作用。这是什么？

Answer 1

你完成了困难的部分，乍一看它看起来是正确的，你想要的是用 lambda 包装它所以像 x => ex 这样的部分：

var parameter = Expression.Parameter(type,"x");
var lambda = Expression.Lambda<Func<TEntity,TEntity>>(ex, parameter);
UnitOfWork.Query<TEntity>( e => e.Id == id )
    .Update( lambda );

(凭记忆写的待验证)

当我有一个表达式时，如何获得 Expression<System.Func<TEntity, TEntity>>？

How do I get Expression<System.Func<TEntity, TEntity>> when I have an Expression?

c#

entity-framework

generics

expression-trees