TypeName

Specifies the type name for the master page.

VirtualPath

Specifies the path to the file that generates the strong type.

Use the @ MasterType directive to set the strong type for the master page, as accessed through the Master property.

Note:
if VirtualPath is not defined, the type must exist in one of the currently linked assemblies, such as App_Bin or App_Code.

If both attributes, TypeName and VirtualPath, are defined, the @ MasterType directive will fail.

The following code example demonstrates how to set the virtual path to an ASP.NET master page.

<%@ MasterType VirtualPath="~/masters/SourcePage.master”" %>

Reference