ASP.NET Core 3.0: The type or namespace name 'CreateDefaultBuilder' does not exist in the namespace

Take another look at the error message:

The type or namespace name 'CreateDefaultBuilder' does not exist in the namespace 'Template.Host'...

When you write Host.CreateDefaultBuilder in a namespace of Template.Host, the compiler assumes you mean Template.Host.CreateDefaultBuilder.

There's a few options for fixing this:

1. Nest the using statement inside of your namespace:

    namespace Template.Host
    {
        using Microsoft.Extensions.Hosting;
   
        // ...
    }
   

2. Alias the Microsoft.Extensions.Hosting.Host type inside of your namespace:

    namespace Template.Host
    {
        using Host = Microsoft.Extensions.Hosting.Host;
   
        // ...
    }
   

3. Use the fully qualified name for the Host type:

    Microsoft.Extensions.Hosting.Host.CreateDefaultBuilder(args)
   

Host represents the Generic Host and is preferred over WebHost in ASP.NET Core 3.0+.