Names of Anonymous Type Members

Recently I was asked how can you get a list of anonymous type member names given a query of anonymous types.  The quick answer is that you can use a quick bit of reflection to get back the names. 

    Public Function GetAnonymousTypeMemberNames(Of T)(ByVal anonymousType As T) As List(Of String)
        Dim type = GetType(T)
        Dim list = New List(Of String)
        For Each cur In type.GetProperties(Reflection.BindingFlags.Public Or Reflection.BindingFlags.Instance)
        Return list
    End Function

The longer question is how can you get back a list of anonymous type member names given a query result?  As long as you know the query will be populated you can just use the first member to get your result.

        Dim q = From it In "astring" Select a = it, b = it & "b"

However you can’t guarantee that a query will always have data in it.  Another route is to consider the contract of the From … Select statement.  This will produce a query which implements IEnumerable(Of T).  For a query T will be the anonymous type that is generated[1].  We can query the metadata of the returned type to get the System.Type for the anonymous type regardless of the type actually implementing IEnumerable(Of T). 

        Dim q = From it In "astring" Select a = it, b = it & "b"
        Dim enumerableInterface = GetType(IEnumerable(Of ))
        Dim enumerableType = q.GetType().GetInterface(enumerableInterface.FullName)
        Dim anonymousTypeType = enumerableType.GetGenericArguments(0)

[1] This is assuming that you didn’t write a query which returns a field directly and avoids creating the anonymous type. 

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