linq - 使用 LINQ 动态映射(或构造投影)

Question

我知道我可以使用 LINQ 使用投影映射两种对象类型:

var destModel = from m in sourceModel
               select new DestModelType {A = m.A, C = m.C, E = m.E}

在哪里

class SourceModelType
{
    string A {get; set;}
    string B {get; set;}
    string C {get; set;}
    string D {get; set;}
    string E {get; set;}
}

class DestModelType
{
    string A {get; set;}
    string C {get; set;}
    string E {get; set;}
}

但是如果我想做一些类似泛型的东西来做这件事，我不知道我正在处理的两种具体类型。所以它会走“Dest”类型并匹配匹配的“Source”类型..这可能吗？此外，为了实现延迟执行，我希望它只返回一个 IQueryable。

例如:

public IQueryable<TDest> ProjectionMap<TSource, TDest>(IQueryable<TSource> sourceModel)
{
   // dynamically build the LINQ projection based on the properties in TDest

   // return the IQueryable containing the constructed projection
}

我知道这很有挑战性，但我希望并非不可能，因为它将为我省去大量模型和 View 模型之间的显式映射工作。

Answer 1

你必须生成一个表达式树，但是一个简单的树，所以并不难......

void Main()
{
    var source = new[]
    {
        new SourceModelType { A = "hello", B = "world", C = "foo", D = "bar", E = "Baz" },
        new SourceModelType { A = "The", B = "answer", C = "is", D = "42", E = "!" }
    };

    var dest = ProjectionMap<SourceModelType, DestModelType>(source.AsQueryable());
    dest.Dump();
}

public static IQueryable<TDest> ProjectionMap<TSource, TDest>(IQueryable<TSource> sourceModel)
    where TDest : new()
{
    var sourceProperties = typeof(TSource).GetProperties().Where(p => p.CanRead);
    var destProperties =   typeof(TDest).GetProperties().Where(p => p.CanWrite);
    var propertyMap = from d in destProperties
                      join s in sourceProperties on new { d.Name, d.PropertyType } equals new { s.Name, s.PropertyType }
                      select new { Source = s, Dest = d };
    var itemParam = Expression.Parameter(typeof(TSource), "item");
    var memberBindings = propertyMap.Select(p => (MemberBinding)Expression.Bind(p.Dest, Expression.Property(itemParam, p.Source)));
    var newExpression = Expression.New(typeof(TDest));
    var memberInitExpression = Expression.MemberInit(newExpression, memberBindings);
    var projection = Expression.Lambda<Func<TSource, TDest>>(memberInitExpression, itemParam);
    projection.Dump();
    return sourceModel.Select(projection);
}

(在 LinqPad 中测试，因此是 Dump)

生成的投影表达式如下所示:

item => new DestModelType() {A = item.A, C = item.C, E = item.E}