c# - 为其中包含嵌套实体的实体生成动态选择表达式

Question

我有一个类，它具有类型为另一个实体的属性。如下所示:

public class InspectionTopicDataAllViewModel 
{
    public Guid Id { get; set; }
    public Guid? UploadedFileId { get; set; }
    public int InspectionId { get; set; }
    public int InspectionTopicId { get; set; }

    public SparseDataViewModel SparseData { get; set; }
}

public class SparseDataViewModel : SparseData
{        
}

我需要编写一个能够只选择我想要的列的动态选择表达式。

我这样写:

private static Func<InspectionTopicDataAll, InspectionTopicDataAllViewModel> GetSparseInitExpression(List<string> columns)
    {
        Expression<Func<InspectionTopicDataAll, InspectionTopicDataAllViewModel>> commonSelector =
           x => new InspectionTopicDataAllViewModel()
           {
               Id = x.Id,
               InspectionId = x.InspectionId,
               InspectionTopicId = x.InspectionTopicId,
               UploadedFileId = x.UploadedFileId,
           };


        // input parameter "x"
        var xParameter = Expression.Parameter(typeof(InspectionTopicDataAll), "x");           

        var xNew = Expression.New(typeof(InspectionTopicDataAllViewModel));

        var sNew = Expression.New(typeof(SparseDataViewModel));

        var bindings = new List<MemberBinding>();

        // create initializers
        foreach (var column in columns)
        {               
            Expression srcBody = xParameter;

            foreach (var member in column.Split('.'))
            {
                srcBody = Expression.PropertyOrField(srcBody, member);                
            }

            var destMember = srcBody as MemberExpression;
            // property "Field1"
            var propInfo = destMember?.Member as PropertyInfo;

            if (propInfo != null)
            {

                bindings.Add(Expression.Bind(propInfo, srcBody));
            }                               
        }

        var sInit = Expression.MemberInit(sNew, bindings);                                  

        var zeroth = ((MemberInitExpression)commonSelector.Body);
        var param = commonSelector.Parameters[0];
        List<MemberBinding> newBindings = new List<MemberBinding>(zeroth.Bindings.OfType<MemberAssignment>());

        var spNestedType = typeof (InspectionTopicDataAllViewModel).GetProperty(nameof(SparseData));

        newBindings.Add(Expression.Bind(spNestedType, sInit));

        var newInit = Expression.MemberInit(xNew, newBindings);

        var childSelector = Expression.Lambda<Func<InspectionTopicDataAll, InspectionTopicDataAllViewModel>>(newInit, xParameter);

        return childSelector.Compile();
}

并在我的主要查询中使用如下函数:

 var model = worksheetService.GetInspectionTopicData(inspectionTopicId, inspectionId, discrimintor, value).Select(GetSparseInitExpression(columns)).AsQueryable();

使用这个函数，我可以生成如下所需的表达式:

x => new InspectionTopicDataAllViewModel() 
{
    Id = x.Id, 
    InspectionId = x.InspectionId,
    InspectionTopicId = x.InspectionTopicId, 
    UploadedFileId = x.UploadedFileId, 
    SparseData = new SparseDataViewModel() {
        MusteriNo = x.SparseData.MusteriNo, 
        MusteriAdi = x.SparseData.MusteriAdi, 
        CekNo = x.SparseData.CekNo, 
        Banka = x.SparseData.Banka, 
        CekTarihi = x.SparseData.CekTarihi, 
        Meblag = x.SparseData.Meblag, 
        PCN = x.SparseData.PCN, 
        Kesideci = x.SparseData.Kesideci}}}

但是当我运行表达式时抛出这个错误:

variable 'x' of type 'InternalControl.Domain.Entities.InspectionTopicDataAll' referenced from scope '', but it is not defined

我不知道如何解决这个错误...

更新:感谢@MBoros 的帮助，对于那些以后可能需要此类代码的人，使用上面的代码，即使代码有效并且可以从数据库中检索结果，但 select 语句不是按预期工作，所有列都从数据库中获取，所以为了解决这个问题，我使用了下面的代码:希望能帮助其他尝试做这种事情的人。

 private static Expression<Func<InspectionTopicDataAll, InspectionTopicDataAllViewModel>> GetCustomSelectExpression(List<string> columns)
    {
        Expression<Func<InspectionTopicDataAll, InspectionTopicDataAllViewModel>> commonSelector =
           x => new InspectionTopicDataAllViewModel()
           {
               Id = x.Id,
               InspectionId = x.InspectionId,
               InspectionTopicId = x.InspectionTopicId,
               UploadedFileId = x.UploadedFileId,
           };


        // input parameter "x"                    
        var xParameter = commonSelector.Parameters[0];

        var xNew = Expression.New(typeof(InspectionTopicDataAllViewModel));

        var sNew = Expression.New(typeof(SparseDataViewModel));

        var bindings = new List<MemberBinding>();

        // create initializers
        foreach (var column in columns)
        {               
            Expression srcBody = xParameter;

            foreach (var member in column.Split('.'))
            {
                srcBody = Expression.PropertyOrField(srcBody, member);                
            }

            var destColName = column.Substring(column.LastIndexOf('.')+1);               
            // property "Field1"
            var propInfo = typeof(SparseDataViewModel).GetProperty(destColName);


            if (propInfo != null)
            {
                bindings.Add(Expression.Bind(propInfo, srcBody));
            }                               
        }

        var sInit = Expression.MemberInit(sNew, bindings);                                  

        var zeroth = ((MemberInitExpression)commonSelector.Body);

        List<MemberBinding> newBindings = new List<MemberBinding>(zeroth.Bindings.OfType<MemberAssignment>());

        var spNestedType = typeof (InspectionTopicDataAllViewModel).GetProperty(nameof(SparseData));

        newBindings.Add(Expression.Bind(spNestedType, sInit));

        var newInit = Expression.MemberInit(xNew, newBindings);

        var childSelector = Expression.Lambda<Func<InspectionTopicDataAll, InspectionTopicDataAllViewModel>>(newInit, xParameter);

        return childSelector;
    } 

并使用它代替如下的 select 语句:

var model =
            worksheetService.GetInspectionTopicData(inspectionTopicId, inspectionId, discrimintor, value)
                .Select(GetCustomSelectExpression(columns));

Answer 1

您的 childSelector 不知道 commonSelector 的 x 参数。在表达式树中，参数是按实例识别的，而不仅仅是按名称识别的，因此两个 x 参数实际上是不同的(即使在调试 View 中它们看起来相同)。

最简单的解决办法可能是改变你的

// input parameter "x"
var xParameter = Expression.Parameter(typeof(InspectionTopicDataAll), "x"); 

到

var xParameter = commonSelector.Parameters[0]; 

这应该可以解决问题:)