Set RenderFragment default content in code behind

Question

I have the following code from an article that is placed inside a .razor file. How can the wig-pig code part be converted to a code behind .razor.cs file?

[Parameter] public RenderFragment<TItem> ItemTemplate { get; set; }

protected override void OnParametersSet()
  {
    if (ItemTemplate == null)
    {
      ItemTemplate = (item) => @:@{ 
      <li @key=item>@item.ToString()</li>}
      ;
    }
}

Answer 1

As per Mister Magoo's comment:

    [Parameter]
    public RenderFragment<TItem> ItemTemplate { get; set; }
            = (TItem item) => (builder) =>
            {
                builder.OpenElement(0, "li");
                builder.SetKey(item);
                builder.AddContent(1, item.ToString());
                builder.CloseElement();
            };

Answer 2

A bit opinionated but I think you are exaggerating the 'code-behind concept' a little.

An ItemTemplate will only see use in the markup section, I would opt for a (small) @code section:

@code {
  [Parameter] 
  public RenderFragment<TItem> ItemTemplate { get; set; } =
         item => @<li @key=item>@item.ToString()</li>;
}