To access members of a specific master page from a content page, you can create a strongly typed reference to the master page by creating a @MasterType directive. This directive supports of two attributes such as TypeName and VirtualPath.
@ MasterType
Provides a way to create a strongly typed reference to the ASP.NET master page when the master page is accessed from the Master property.
<%@ MasterType attribute="value" [attribute="value"...] %>
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.
MasterType Directive in ASP.NET
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