code behind in newform.Aspx?

I have a list that must be filled with a bit of code behind (prepopulating some fields, and do a bit of work on the save button.)

What is the best way to do that ?

thx

Edit: I ended by creating a custom webpart on the default.aspx. In this web part I have a bunch of :

<table border="0" cellspacing="0" width="100%">
<tr>
    <td width="190px" valign="top" class="ms-formlabel">
        <h3 class="ms-standardheader">
            <nobr>Title<span class="ms-formvalidation"> *</span>
            </nobr>
        </h3>
    </td>
    <td width="400px" valign="top" class="ms-formbody">
        <SharePoint:FormField runat="server" id="fldTitle" ControlMode="New" FieldName="Title" ListId="{MyListID}" />
        <SharePoint:FieldDescription runat="server" id="ff1description" FieldName="Title" ControlMode="New" ListId="{MyList开发者_如何转开发ID}"/>
    </td>
</tr>

This is working, but I found this in a bit painfull because I have to read each form field in code behind :

    private void Set(SPListItem item, string fieldInternalName, object fieldValue)
    {
        var field = item.Fields.GetFieldByInternalName(fieldInternalName);
        item[fieldInternalName] = fieldValue;
    }
    protected void Btn_Ok_Click(object sender, EventArgs e)
    {

        SPWeb thisWeb = SPContext.Current.Web;
        SPList myList= thisWeb.Lists["mylist"];
        SPListItem newItem;
        newItem= myList.Items.Add();

        var router = thisWeb.EnsureUser(@"myuser");

        Set(newItem, "Title", fldTitle.Value);
        Set(newItem, "OtherField", fldOther.Value);
        Set(newItem, "AnotherField", GetFromBusinessLogic());


        SPUtility.Redirect(thisWeb.Url, SPRedirectFlags.Default, System.Web.HttpContext.Current);
        newItem.Update();

    }

Is there any way to wrap all of this in a custom form container ? Maybe a custom ListFormWebPart with inner templates and code-behind events ?

You can follow the suggestion here to add in the code-behind.

BUT even if you put in a code-behind file, you still cannot reference the controls (fields, save buttons etc) on the form from the code-behind.

I would suggest writing up a web service (asmx or WCF) and host it on the Sharepoint instance. You can then use javascript and AJAX calls to perform initialization and validation.