Hello Stanislas,

Thank you for posting in our community.

Setting the multiline mode for the igGrid editor in MVC could be achieved via the ColumnSetting collection of the Updating feature. For example:

  @(Html.Infragistics().Grid(Model)
  .ID("grid1")
  .Height("300px")
  .AutoGenerateColumns(false)
  .PrimaryKey("ProductID")
  .Columns(col =>
{
    col.For(c => c.ProductID).HeaderText("ProductID");
    col.For(c => c.Name).HeaderText("Name");
    col.For(c => c.ReleaseDate).HeaderText("ReleaseDate");
})
.Features(features =>
{
    features.Updating().EditMode(GridEditMode.Row).ColumnSettings(cs => {
        cs.ColumnSetting().ColumnKey("Name").EditorOptions("textMode : 'multiline'");        
    });
   
})
.DataSourceUrl(Url.Action("GetData"))
.Render())

I hope you find this information helpful.

Please let me know if you need any further assistance with this matter.