Public Property ReadOnlyField As Boolean

Dim instance As SPField
Dim value As Boolean

value = instance.ReadOnlyField

instance.ReadOnlyField = value

public bool ReadOnlyField { get; set; }

Using oSiteCollection As New SPSite("http://MyServer")
    Using oWebsite As SPWeb = oSiteCollection.OpenWeb()
        Dim oList As SPList = oWebsite.Lists("MyList")
        Dim oItem As SPListItem = oList.Items(0)
        oItem.Fields("MyField").ReadOnlyField = True
        oItem.Fields("MyField").Update()
    End Using
End Using

using (SPSite oSiteCollection = new SPSite("http://MyServer"))
{
    using (SPWeb oWebsite = oSiteCollection.OpenWeb())
    {
        SPList oList = oWebsite.Lists["MyList"];
        SPListItem oItem = oList.Items[0];
        oItem.Fields["MyField"].ReadOnlyField = true;
        oItem.Fields["MyField"].Update();
    }
}