C# – Why does this error occur when using SingleAsync

asp.net-corecentity-frameworklinq

Find if an item is duplicated.
Expected: an exception to be thrown only if more than one item is found. but we get a different exception here?

        try
        {
            // Find duplicate item
            duplicateItem = await _context.Items
                .SingleAsync(m => m.UserId == userId && m.Name == "some item");
        }
        catch (Exception)
        {
    //Exception more than one item found
        }

[01:48:06 ERR] An exception occurred while iterating over the results of a query for context type '….ApplicationDbContext'.
System.InvalidOperationException: Source sequence doesn't contain any elements.
at System.Linq.AsyncEnumerable.Single_[TSource](IAsyncEnumerable1 source, CancellationToken cancellationToken) in D:\a\1\s\Ix.NET\Source\System.Interactive.Async\Single.cs:line 136
at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.TaskResultAsyncEnumerable1.Enumerator.MoveNext(CancellationToken cancellationToken)
at System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator2.MoveNextCore(CancellationToken cancellationToken) in D:\a\1\s\Ix.NET\Source\System.Interactive.Async\Select.cs:line 106
at System.Linq.AsyncEnumerable.AsyncIterator1.MoveNext(CancellationToken cancellationToken) in D:\a\1\s\Ix.NET\Source\System.Interactive.Async\AsyncIterator.cs:line 98
at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor1.EnumeratorExceptionInterceptor.MoveNext(CancellationToken cancellationToken)
at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteSingletonAsyncQuery[TResult](QueryContext queryContext, Func2 compiledQuery, IDiagnosticsLogger`1 logger, Type contextType)

System.InvalidOperationException: Source sequence doesn't contain any
elements. at
System.Linq.AsyncEnumerable.Single_[TSource](IAsyncEnumerable1
source, CancellationToken cancellationToken) in
D:\a\1\s\Ix.NET\Source\System.Interactive.Async\Single.cs:line 136
at
Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.TaskResultAsyncEnumerable1.Enumerator.MoveNext(CancellationToken
cancellationToken) at
System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator2.MoveNextCore(CancellationToken
cancellationToken) in
D:\a\1\s\Ix.NET\Source\System.Interactive.Async\Select.cs:line 106
at
System.Linq.AsyncEnumerable.AsyncIterator1.MoveNext(CancellationToken
cancellationToken) in
D:\a\1\s\Ix.NET\Source\System.Interactive.Async\AsyncIterator.cs:line
98 at
Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor1.EnumeratorExceptionInterceptor.MoveNext(CancellationToken
cancellationToken) at
Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteSingletonAsyncQuery[TResult](QueryContext
queryContext, Func2 compiledQuery, IDiagnosticsLogger`1 logger, Type
contextType)

Best Answer

You are confusing SingleAsync with SingleOrDefaultAsync.

SingleAsync -> means that it will throw if no entries or more than one entry is found

Asynchronously returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.

SingleOrDefaultAsync -> means that it will throw if two or more entries are found

Asynchronously returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

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