I find the way to access the InfoPath Controls in C#
Code to read Form field data using c#:
XPathNavigator elemYourTextBox = this.MainDataSource.CreateNavigator()
.SelectSingleNode("//my:YourTextBox", this.NamespaceManager);
String DataFromTextField = elemYourTextBox.Value;
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Code to set Form field data using c#:
XPathNavigator elemYourTextBox = this.MainDataSource.CreateNavigator()
.SelectSingleNode("//my:YourTextBox", this.NamespaceManager);
elemYourTextBox.SetValue("Your String Data");