System.String ElementAt[String](System.Collections.Generic.IEnumerable`1[System.String], Int32)' method

Question

I am recieving the exception mentioned in quotes below. I have tried to remove the problem alot but i am getting the exception below. I have read similiar answers to my problem in stack over flow but they don't solve my problem. I have also commented above the line where i recieve exception.

System.NotSupportedException was unhandled by user code
HResult=-2146233067 Message=LINQ to Entities does not recognize the method 'System.String ElementAt[String](System.Collections.Generic.IEnumerable1[System.String], Int32)' method, and this method cannot be translated into a store expression. Source=EntityFramework StackTrace: at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.DefaultTranslator.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.TypedTranslator1.Translate(ExpressionConverter parent, Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.UnaryTranslator.TypedTranslate(ExpressionConverter parent, UnaryExpression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator1.Translate(ExpressionConverter parent, Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.EqualsTranslator.TypedTranslate(ExpressionConverter parent, BinaryExpression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TypedTranslator1.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.TypedTranslator1.Translate(ExpressionConverter parent, Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) 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.SelectTranslator.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.TypedTranslator1.Translate(ExpressionConverter parent, Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateExpression(Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.TranslateSet(Expression linq) at System.Data.Entity.Core.Objects.ELinq.ExpressionConverter.MethodCallTranslator.UnarySequenceMethodTranslator.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.TypedTranslator1.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(Nullable1 forMergeOption) at System.Data.Entity.Core.Objects.ObjectQuery1.<>c__DisplayClass7.<GetResults>b__6() at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess) at System.Data.Entity.Core.Objects.ObjectQuery1.<>c__DisplayClass7.<GetResults>b__5() at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func1 operation) at System.Data.Entity.Core.Objects.ObjectQuery1.GetResults(Nullable1 forMergeOption) at System.Data.Entity.Core.Objects.ObjectQuery1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__0() at System.Data.Entity.Internal.LazyEnumerator1.MoveNext() at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable1 source) at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__1[TResult](IEnumerable1 sequence) at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable1 query, Expression queryRoot) at System.Data.Entity.Core.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[TResult](Expression expression) at System.Data.Entity.Internal.Linq.DbQueryProvider.Execute[TResult](Expression expression) at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable1 source) at Neon.DBHandler.registerStudentCourses(String p, List`1 coursesIdSelected) in c:\Users\AbdullahABBASI\Desktop\AbbasiWebProject\Neon\Models\DBHandler.cs:line 246 at Neon.Student_RegisterCourse.registerbutton_Click(Object sender, EventArgs e) in c:\Users\AbdullahABBASI\Desktop\AbbasiWebProject\Neon\Student_RegisterCourse.aspx.cs:line 51 at System.Web.UI.WebControls.Button.OnClick(EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
InnerException:

Code is below:

internal static void registerStudentCourses(string p, List<string> coursesIdSelected)
{
    AbbasiDatabaseEntities objDB = new AbbasiDatabaseEntities();
    Student objStudent = objDB.Students.Where(s => s.Id == p).FirstOrDefault();
    List<Course> courses = new List<Course>();

    for (int i = 0; i<coursesIdSelected.Count; i++)
    {
        //in the line below i am getting exception mentioned above
        var course = objDB.Courses
            .Where(c => c.Id == (string)coursesIdSelected.ElementAt(i))
            .Select(s=> s)
            .FirstOrDefault();
        courses.Add(course);
    }

    objStudent.Courses = courses;    
}

LINQ query is converted to SQL at runtime. For this reason, .ElementAt(i) is not accessible — PaulShovan
– PaulShovan, Commented May 26, 2015 at 12:18
You are doing a SELECT N+1 (technically this is a SELECT N)... in general this is an antipattern (you are doing a select in a for cycle, so there will be n separate queries executed to the db) — xanatos
– xanatos, Commented May 26, 2015 at 12:20

CodeCaster · Accepted Answer · 2015-05-26 12:20:28Z

8

Entity Framework can't convert Where(c => c.Id == (string)coursesIdSelected.ElementAt(i)) to a query.

Introduce a helper variable to remove that code from your query:

for (int i = 0; i<coursesIdSelected.Count; i++){
    string selectedCourse = (string)coursesIdSelected.ElementAt(i);
    var course = objDB.Courses.Where(c => c.Id == selectedCourse).Select(s=> s).FirstOrDefault();
    courses.Add(course);
}

Or simplify the entire loop to:

student.Courses = db.Courses.Where(c => coursesIdSelected.Contains(c.Id.ToString()));

edited May 26, 2015 at 12:20

answered May 26, 2015 at 12:18

CodeCaster

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3 Comments

xanatos Over a year ago

Considering List<string> coursesIdSelected, the ElementAt() could be changed to a [] (and then perhaps it would work directly).

xanatos Over a year ago

Better the second one... It solves the antipattern of doing a select inside a for

Abdullah Jahangir Abbasi Over a year ago

Best Solution Ever Lovely abbasi Loves You

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