/// <summary>
/// Creates a new Global Parameter of type Length, assigns it an initial value,
/// and uses it to label a set of input dimension elements.
/// </summary>
/// <param name="document">Revit project document in which to create the parameter.</param>
/// <param name="name">Name of the global parameter to create.</param>
/// <param name="value">A value the new global parameter is to have.</param>
/// <param name="dimensionsToLabel">A set of dimension to labe by the new global parameter.</param>
/// <returns>ElementId of the new GlobalParameter</returns>
public ElementId CreateNewGlobalParameter(Document document, String name, double value, ISet<ElementId> dimensionsToLabel)
{
    if (!GlobalParametersManager.AreGlobalParametersAllowed(document))
        throw new System.InvalidOperationException("Global parameters are not permitted in the given document");

    if (!GlobalParametersManager.IsUniqueName(document, name))
        throw new System.ArgumentException("Global parameter with such name already exists in the document", "name");

    ElementId gpid = ElementId.InvalidElementId;

    // creation of any element must be in a transaction
    using (Transaction trans = new Transaction(document, "Create Global Parameter"))
    {
        trans.Start();

        // create a GP with the given name and type Length
        GlobalParameter gp = GlobalParameter.Create(document, name, ParameterType.Length);
        if (gp != null)
        {
            // if created successfully, assign it a value
            // note: parameters of type Length accept Double values
            gp.SetValue(new DoubleParameterValue(value));

            // if a collection of dimensions was given, label them with this new parameter
            foreach (ElementId elemid in dimensionsToLabel)
            {
                // not just any dimension is allowed to be labeled
                // check first to avoid exceptions
                if (gp.CanLabelDimension(elemid))
                {
                    gp.LabelDimension(elemid);
                }
            }

            gpid = gp.Id;
        }
        trans.Commit();
    }

    return gpid;
}

' <summary>
' Creates a new Global Parameter of type Length, assigns it an initial value,
' and uses it to label a set of input dimension elements.
' </summary>
' <param name="document">Revit project document in which to create the parameter.</param>
' <param name="name">Name of the global parameter to create.</param>
' <param name="value">A value the new global parameter is to have.</param>
' <param name="dimensionsToLabel">A set of dimension to labe by the new global parameter.</param>
' <returns>ElementId of the new GlobalParameter</returns>
Public Function CreateNewGlobalParameter(document As Document, name As [String], value As Double, dimensionsToLabel As ISet(Of ElementId)) As ElementId
    If Not GlobalParametersManager.AreGlobalParametersAllowed(document) Then
        Throw New System.InvalidOperationException("Global parameters are not permitted in the given document")
    End If

    If Not GlobalParametersManager.IsUniqueName(document, name) Then
        Throw New System.ArgumentException("Global parameter with such name already exists in the document", "name")
    End If

    Dim gpid As ElementId = ElementId.InvalidElementId

    ' creation of any element must be in a transaction
    Using trans As New Transaction(document, "Create Global Parameter")
        trans.Start()

        ' create a GP with the given name and type Length
        Dim gp As GlobalParameter = GlobalParameter.Create(document, name, ParameterType.Length)
        If gp IsNot Nothing Then
            ' if created successfully, assign it a value
            ' note: parameters of type Length accept Double values
            gp.SetValue(New DoubleParameterValue(value))

            ' if a collection of dimensions was given, label them with this new parameter
            For Each elemid As ElementId In dimensionsToLabel
                ' not just any dimension is allowed to be labeled
                ' check first to avoid exceptions
                If gp.CanLabelDimension(elemid) Then
                    gp.LabelDimension(elemid)
                End If
            Next

            gpid = gp.Id
        End If
        trans.Commit()
    End Using

    Return gpid
End Function