How do I write Blazor HTML code inside the @code block?

Version 1

In Blazor idiomatic way would be create component instead of attempting to write HTML directly in the @code.

Create drawSomething.razor

<p>@Message</p>

@code {
    [Parameter]
    public string Message {get;set;}
}

and in your Test.razor

@page "/Test"

@if (option == 1)
{
    <drawSomething Message="Something" />
}
else
{
    <drawSomething Message="Something else" />
}

@code {
    int option;
}

Here I assuming that you have something more complex, then just plain

.

Version 2

If you really want easy way, then just

@page "/Test"

@if (option == 1)
{
    <p>Something</p>
}
else
{
    <p>Something else</p>
}

@code {
    int option;
}

Version 3 Based on suggestion from Isaac

@page "/Test"

@if (option == 1)
{
    <drawSomething Message="Something" />
}
else
{
    <drawSomething Message="Something else" />
}

@code {
    int option;

    RenderFragment drawSomething(string message)
    {
        return @<p>@message</p>;
    }
}

Update, you can now use:

@GreetPerson("John")

@code {
  RenderFragment GreetPerson(string name)
  {
    return @<p>Hello <em>@name</em></p>;
  }
}

Old answer:

This was announced as a feature for Preview6 but it didn't work as advertised then, and some details seem to have changed later. There is a comment from Cosmin Sontu at the bottom of that page that points the right way:

@using Microsoft.AspNetCore.Components.Rendering

@*tested with preview 9*@
@{ GreetPerson(__builder, "John"); }

@code {

    void GreetPerson(RenderTreeBuilder __builder, string name)
    {            
        <p>Hello, <em>@name!</em></p>
    }
}

The name __builder cannot be changed. That is a double underscore.