c# - NotSupportedException 在 LINQ 和 EntityFramework 中使用 ToList()

Question

我有一个如下所示的查询:

    var q = from x in db.Table1
                where x.Timestamp.CompareTo(fromDate) >= 0
                   && x.Timestamp.CompareTo(toDate) < 0
                select x;

    var q_list = q.ToList(); // this works fine
    var g = q.Where(z=> z.Table2.Equals(ns));  // ns is instance of Table2 not Table1
    var g_list = g.ToList(); // this throws exception 

异常:

**

System.NotSupportedException was unhandled
  Message=Unable to create a constant value of type ...

我有很多这样的功能，我不想重复查询，我需要使用 Where，GroupBy ...等功能。

完整异常跟踪:

**

System.NotSupportedException was unhandled
  Message=Unable to create a constant value of type 'DataAccessLayer.Model.Table2. Only primitive types ('such as Int32, String, and Guid') are supported in this context.
  Source=System.Data.Entity
  StackTrace:
       at System.Data.Objects.ELinq.ExpressionConverter.ConstantTranslator.TypedTranslate(ExpressionConverter parent, ConstantExpression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.EqualsTranslator.TypedTranslate(ExpressionConverter parent, BinaryExpression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input, DbExpressionBinding& binding)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)
       at System.Data.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
       at System.Data.Objects.ELinq.ExpressionConverter.Convert()
       at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
       at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
       at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
       at System.Data.Entity.Internal.Linq.InternalQuery`1.GetEnumerator()
       at System.Data.Entity.Infrastructure.DbQuery`1.System.Collections.Generic.IEnumerable<TResult>.GetEnumerator()
       at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)

**

Answer 1

问题是 EntityFramework 不理解非原始类型。你要做的是:

var g = q.Where(z => z.field.Id.Equals(ns.Id)); 

请记住，如果您有 Equals 的自定义实现，则无法将其转换为正确的 t-sql。如果您无论如何都想执行该实现，则需要在内存中执行:

// note it's using q_list instead of q
var g = q_list.Where(z => z.field.Equals(ns));