C＃Linq to Entities方法将属性投影到字符串

Question

我试图重构遍布各处使用的一行代码。我们正在使用EF6.1并希望找到电话和电子邮件（以字符串形式）。

public SiteLabelDto[] GetVendorSites(int vendorId) 
    { 
     return Context.Sites.Where(s => !s.IsDeleted && s.Vendor.Id == vendorId) 
      .Select(s => new SiteLabelDto 
      { 
       Id = s.Id, 
       Name = s.Name,      
       Address = s.Address.StreetAddress + ", " + s.Address.Locality + ", " + s.Address.Region + ", " + s.Address.Postcode, 
       Country = s.Address.Country.Name, 
       Email = s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Email) != null ? s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Email).Value : "", 
       Phone = s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Phone) != null ? s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Phone).Value : "",      
      }).ToArray(); 
    } 

上面的代码获取了接触点列表，并试图找到最适合每种类型的代码。

public class ContactPointEntity 
{ 
    public int Id { get; set; } 
    public string Value { get; set; } 
    public ContactPointType Type { get; set; } 
    public bool IsDefault { get; set; } 
} 

该方法将被扩展为尝试返回第一个IsDefault。

我的目标是尝试并能够投入的方法或扩展，这样我可以说s.GetcontactPoint（ContactPointType.Email）或s.contactPoints.GetPoints（ContactPointType.Email）并返回字符串值，或者如果字符串不是可能的情况，则返回联系点类。

我越读它，我想我需要建立一些表达式树，不知道现在怎么样。

Answer 1

你需要建立一个表达式树。

首先，因为你需要引入IsDefault条件下，表达可以是这样的：

s.ContactPoints 
.Where(x => x.Type == ContactPointType.Email && x.IsDefault) 
.Select(x => x.Value) 
.DefaultIfEmpty(string.Empty) 
.FirstOrDefault() 

然后，接触点选择转换成一种表达。

private static Expression<Func<Site, string>> GetContactPoint(ParameterExpression siteParam, ParameterExpression cpeParam, ContactPointType type) 
{ 
    // Where. 
    var typeCondition = Expression.Equal(Expression.Property(cpeParam, "Type"), Expression.Constant(type)); 
    var defaultCondition = Expression.Equal(Expression.Property(cpeParam, "IsDefault"), Expression.Constant(true)); 
    var condition = Expression.AndAlso(typeCondition, defaultCondition); 
    var predicateExp = Expression.Lambda<Func<ContactPointEntity, bool>>(condition, cpeParam); 
    var whereExp = Expression.Call(typeof(Enumerable), "Where", new[] { typeof(ContactPointEntity) }, Expression.Property(siteParam, "ContactPoints"), predicateExp); 

    // Select. 
    var valueExp = Expression.Lambda<Func<ContactPointEntity, string>>(Expression.Property(cpeParam, "Value"), cpeParam); 
    var selectExp = Expression.Call(typeof(Enumerable), "Select", new[] { typeof(ContactPointEntity), typeof(string) }, whereExp, valueExp); 

    // DefaultIfEmpty. 
    var defaultIfEmptyExp = Expression.Call(typeof(Enumerable), "DefaultIfEmpty", new[] { typeof(string) }, selectExp, Expression.Constant(string.Empty)); 

    // FirstOrDefault. 
    var firstOrDefaultExp = Expression.Call(typeof(Enumerable), "FirstOrDefault", new[] { typeof(string) }, defaultIfEmptyExp); 

    var selector = Expression.Lambda<Func<Site, string>>(firstOrDefaultExp, siteParam); 
    return selector; 
} 

并且还创建站点标签dto选择器。

private static Expression<Func<Site, SiteLabelDto>> GetSite(ParameterExpression siteParam, ParameterExpression cpeParam) 
{ 
    var newExp = Expression.New(typeof(SiteLabelDto)); 
    var initExp = Expression.MemberInit(
     newExp, 
     Expression.Bind(typeof(SiteLabelDto).GetProperty("Id"), Expression.Lambda<Func<Site, int>>(Expression.Property(siteParam, "Id"), siteParam).Body), 
     Expression.Bind(typeof(SiteLabelDto).GetProperty("Name"), Expression.Lambda<Func<Site, string>>(Expression.Property(siteParam, "Name"), siteParam).Body), 
     /* other basic information */ 
     Expression.Bind(typeof(SiteLabelDto).GetProperty("Email"), GetContactPoint(siteParam, cpeParam, ContactPointType.Email).Body), 
     Expression.Bind(typeof(SiteLabelDto).GetProperty("Phone"), GetContactPoint(siteParam, cpeParam, ContactPointType.Phone).Body) 
     /* other types */ 
    ); 
    var selector = Expression.Lambda<Func<Site, SiteLabelDto>>(initExp, siteParam); 
    return selector; 
} 

用法。

var siteParam = Expression.Parameter(typeof(Site), "s"); 
var cpeParam = Expression.Parameter(typeof(ContactPointEntity), "cpe"); 
var selector = GetSite(siteParam, cpeParam); 
return Context.Sites 
    .Where(s => !s.IsDeleted && s.Vendor.Id == vendorId) 
    .Select(selector) 
    .ToArray(); 

PS：

也许有些上面需要的代码进行重构，这只是给出了基本思路是如何做到这一点。

更新

你也可以创建一个包装类与所有接触点一起包含EF实例。

public class ContactPointExt<T> 
{ 
    public T Instance { get; set; } 
    public string Email { get; set; } 
    public string Phone { get; set; } 
} 

然后改变GetSite为GetContactPoints返回结果为ContactPointExt<T>。

private static Expression<Func<Site, ContactPointExt<T>>> GetContactPoints<T>(ParameterExpression siteParam, ParameterExpression cpeParam) 
{ 
    var type = typeof(ContactPointExt<T>); 
    var newExp = Expression.New(type); 
    var initExp = Expression.MemberInit(
     newExp, 
     Expression.Bind(type.GetProperty("Instance"), siteParam), 
     Expression.Bind(type.GetProperty("Email"), GetContactPoint(siteParam, cpeParam, ContactPointType.Email).Body), 
     Expression.Bind(type.GetProperty("Phone"), GetContactPoint(siteParam, cpeParam, ContactPointType.Phone).Body) 
    ); 
    var selector = Expression.Lambda<Func<Site, ContactPointExt<T>>>(initExp, siteParam); 
    return selector; 
} 

的ContactPointExt<T>结果可以被重新投影到SiteLabelDto与另一Select。从OP

var siteParam = Expression.Parameter(typeof(Site), "s"); 
var cpeParam = Expression.Parameter(typeof(ContactPointEntity), "cpe"); 
var selector = GetContactPoints<Site>(siteParam, cpeParam); 
return Context.Sites 
    .Where(s => !s.IsDeleted && s.Vendor.Id == vendorId) 
    .Select(selector) 
    .Select(s => new SiteLabelDto 
    { 
     Id = s.Instance.Id, 
     Name = s.Instance.Name, 
     Email = s.Email, 
     Phone = s.Phone 
    }) 
    .ToArray(); 

EDIT我们创建了一个包装方法，使这个一点点简单的重复使用，把它在这里只是为了证明别人：

/// <summary> 
    /// Wraps up a each of a query's objects in a ContactPointExt&lt;T&gt; object, providing the default contact point of each type. 
    /// The original query object is accessed via the "Instance" property on each result. 
    /// Assumes that the query object type has a property called ContactPoints - if different, supply the property name as the first argument. 
    /// </summary> 
    /// <typeparam name="T"></typeparam> 
    /// <param name="query"></param> 
    /// <param name="contactPointsPropertyName"></param> 
    /// <returns></returns> 
    public static IQueryable<ContactPointExt<T>> WithContactPointProcessing<T>(this IQueryable<T> query, string contactPointsPropertyName = "ContactPoints") where T : class 
    { 
     var siteParam = Expression.Parameter(typeof(T), "s"); 
     var cpeParam = Expression.Parameter(typeof(ContactPointEntity), "cpe"); 
     var selector = ContactPointHelpers.GetContactPoints<T>(siteParam, cpeParam, contactPointsPropertyName); 
     return query.Select(selector); 
    } 

Answer 2

对Linq-to-entites使用扩展方法有点棘手，因为并不是所有提供者都能理解并转化为相应的后端调用。一个相对安全的办法是采取IQueryable并返回IQueryable它可以解决：

public static IQueryable<SiteDTO> MapToSiteDTO(this IQueryable<Site> query) 
{ 
    return query.Select(s => new SiteLabelDto 
          { 
           Id = s.Id, 
           Name = s.Name,      
           Address = s.Address.StreetAddress + ", " + s.Address.Locality + ", " + s.Address.Region + ", " + s.Address.Postcode, 
           Country = s.Address.Country.Name, 
           Email = s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Email) != null ? s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Email).Value : "", 
           Phone = s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Phone) != null ? s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Phone).Value : "",      
          }); 
} 

然后你把它想：

return Context.Sites.Where(s => !s.IsDeleted && s.Vendor.Id == vendorId) 
      .Select(x => x) 
      .MapToSiteDTO() 
      .ToArray(); 

Answer 3

public SiteLabelDto[] GetVendorSites(int vendorId) 
{ 
    return (from s in Context.Sites 
     where !s.IsDeleted && s.Vendor.Id == vendorId 
     let email = s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Email) 
     let phone = s.ContactPoints.FirstOrDefault(x => x.Type == ContactPointType.Phone) 
     select new SiteLabelDto 
     { 
      Id = s.Id, 
      Name = s.Name,      
      Address = s.Address.StreetAddress + ", " + s.Address.Locality + ", " + s.Address.Region + ", " + s.Address.Postcode, 
      Country = s.Address.Country.Name, 
      Email = email != null ? email.Value : "", 
      Phone = phone != null ? phone .Value : "", 
     }).ToArray(); 
}