Sometimes, it may be necessary to set the “QueryText” attribute of the Content Search Web Part Programmatically :

Here’s the existing structure in “Microsoft.Office.Server.Search.dll” :

In order to override the QueryText attribute, you can override the BeforeSerializeToClient event :

public class CustomContentBySearchWebPart : ContentBySearchWebPart
{
    public CustomContentBySearchWebPart() { }

    protected override void OnLoad(EventArgs e)
    {
        if (this.AppManager != null)
        {
            if (this.AppManager.QueryGroups.ContainsKey(this.QueryGroupName) &&
                this.AppManager.QueryGroups[this.QueryGroupName].DataProvider != null)
            {
                this.AppManager.QueryGroups[this.QueryGroupName].DataProvider.BeforeSerializeToClient +=
                    new BeforeSerializeToClientEventHandler(EnhanceQuery);
            }
        }

        base.OnLoad(e);
    }

    private void EnhanceQuery(object sender, BeforeSerializeToClientEventArgs e)
    {    
        DataProviderScriptWebPart dataProvider = sender as DataProviderScriptWebPart;
        dataProvider.QueryTemplate = “ACTUAL QUERY”;
    }
}