使用 concat 多个表达式构建表达式树时，ToList 抛出异常答案

【问题标题】：ToList thows Exception when building Expression tree using concat multiple Expressions使用 concat 多个表达式构建表达式树时，ToList 抛出异常
【发布时间】：2016-08-17 01:40:30
【问题描述】：

我正在构建一个例程来连接多个实体，以便按字符串进行过滤。我使用以下代码创建我的表达式，因为我使用相同的代码进行排序，所以它可以正常工作。

private void SetExpression<T>(string property, ref Type type, ref ParameterExpression arg, ref Expression expr)
{
    string[] props = property.Split('.');
    foreach (string prop in props)
    {
        PropertyInfo pi = type.GetProperty(prop);
        expr = Expression.Property(expr, pi);
        type = pi.PropertyType;
    }
}

然后我使用另外 2 种方法来构建树。我使用 GenerateConcat 来制作连接部分。

private Expression<Func<T, string>> GenerateConcat(ParameterExpression parameterExpression, params Expression[] expressions)
{
    var parameter = parameterExpression;
    var separator = Expression.Constant(" ");
    var concatArgs = expressions;

    var concatCall = Expression.Call(typeof(string).GetMethod("Concat", new[] { typeof(string), typeof(string), typeof(string) }), concatArgs);
    return Expression.Lambda<Func<T, string>>(concatCall, parameter);
}

我使用 GenerateContains 来创建 contains 方法。

private Expression<Func<T, bool>> GenerateContains(Expression<Func<T, string>> member, string value)
{
    var containsCall = Expression.Call(member.Body, "Contains", Type.EmptyTypes, Expression.Constant(value));
    return Expression.Lambda<Func<T, bool>>(containsCall, member.Parameters);
}

运行时，一切正常，我的谓词看起来正确形成。但是，当调用 ToList() 时会引发以下异常。

System.InvalidOperationException: The parameter 'x' was not bound in the specified LINQ to Entities query expression.

Result StackTrace:  
at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.ParameterTranslator.TypedTranslate(ExpressionConverter parent, ParameterExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MemberAccessTranslator.TypedTranslate(ExpressionConverter parent, MemberExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MemberAccessTranslator.TypedTranslate(ExpressionConverter parent, MemberExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.StringTranslatorUtil.ConvertToString(ExpressionConverter parent, Expression linqExpression)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.StringTranslatorUtil.<>c__DisplayClass1c.<ConcatArgs>b__1a(Expression arg)
   at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.StringTranslatorUtil.ConcatArgs(ExpressionConverter parent, Expression linq, Expression[] linqArgs)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.StringConcatTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateFunctionIntoLike(MethodCallExpression call, Boolean insertPercentAtStart, Boolean insertPercentAtEnd, Func`5 defaultTranslator)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.StringContainsTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.BinaryTranslator.TypedTranslate(ExpressionConverter parent, BinaryExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.BinaryTranslator.TypedTranslate(ExpressionConverter parent, BinaryExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateLambda(LambdaExpression lambda, DbExpression input, DbExpressionBinding& binding)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, DbExpression& source, DbExpressionBinding& sourceBinding, DbExpression& lambda)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.OneLambdaTranslator.Translate(ExpressionConverter parent, MethodCallExpression call)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.SequenceMethodTranslator.Translate(ExpressionConverter parent, MethodCallExpression call, SequenceMethod sequenceMethod)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.TypedTranslate(ExpressionConverter parent, MethodCallExpression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator`1.Translate(ExpressionConverter parent, Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq)
   at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.Convert()
   at System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__6()
   at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__5()
   at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func`1 operation)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   at System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__0()
   at System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Triton.Repository.Test.UnitTest1.ComplexTypeTest() in C:\BitBucket\triton-mvc-5\Triton\Triton.Repository.Test\UnitTest1.cs:line 56
Result Message: 
Test method Triton.Repository.Test.UnitTest1.ComplexTypeTest threw exception: 
System.InvalidOperationException: The parameter 'x' was not bound in the specified LINQ to Entities query expression.

我猜，但问题是我如何构建 Concat 部分 (GenerateConcat)，但我似乎无法弄清楚出了什么问题。我是否遗漏了关于 ParameterExpression 的任何内容？我确保所有表达式都相同。提前致谢。

编辑 1：这是创建表达式的代码

我使用 Initialize 块根据属性名称创建表达式。然后我使用 Append 方法从其他属性中获取表达式。我的错误是当我创建子属性时我没有正确分配 ParameterExpression。所以我重载了 Intialize 并添加了一个 ParamerterExpression 参数以保持所有子属性的表达式相同。这样就解决了。

public ExpressionBuilder Initialize<T>(string property)
{
    if (string.IsNullOrEmpty(property))
        throw new ArgumentNullException(property);

    Type type = typeof(T);
    ParameterExpression arg = Expression.Parameter(type, "x");
    Expression expr = arg;

    SetExpression<T>(property, ref type, ref arg, ref expr);

    ExpressionModel = new ExpressionModel() { Expression = expr, ParameterExpression = arg, Type = type };

    IsIntialized = true;
    return this;
}



public ExpressionBuilder Append<T>(string property)
{
    if (string.IsNullOrEmpty(property))
        throw new ArgumentNullException(property);

    if (IsIntialized == false)
        throw new Exception("ExpressionBuilder has not been initialized.");

    Type type = ExpressionModel.Type;
    ParameterExpression arg = ExpressionModel.ParameterExpression;
    Expression expr = ExpressionModel.Expression;

    SetExpression<T>(property, ref type, ref arg, ref expr);
    ExpressionModel.Expression = expr;
    ExpressionModel.ParameterExpression = arg;
    ExpressionModel.Type = type;

    return this;
}

【问题讨论】：

我很难过，但如果你包含调用代码可能会对其他人有所帮助，这可能是x 的来源？
GenerateConcat 有问题（您使用 3 个字符串参数绑定到硬编码重载），但这不会导致此特定异常。您有一些未绑定的参数x（很可能来自外部 lambda）。如果没有重现问题的调用代码，很难准确判断。
感谢您指出正确的方向。

标签： c# linq linq-to-entities expression-trees

【解决方案1】：

子属性的 ParameterExpression 必须与父属性相同。因此，我重载了创建 Expression 的函数，以便能够显式设置 ParameterExpression。

public ExpressionBuilder Initialize<T>(string property, ParameterExpression parameterExpression)
{
    if (string.IsNullOrEmpty(property))
        throw new ArgumentNullException(property);

    Type type = typeof(T);
    ParameterExpression arg = parameterExpression;
    Expression expr = arg;

    SetExpression<T>(property, ref type, ref arg, ref expr);

    ExpressionModel = new ExpressionModel() { Expression = expr, ParameterExpression = arg, Type = type };

    IsIntialized = true;
    return this;
}

【讨论】：