public string DisabledTooltip {get; set;}

//Set the tool's loadOnClick attribute to "false" in the config.daml. 
//This will allow the tool to be created when Pro launches, so that the disabledText property can display customized text at startup.
//Remove the "condition" attribute from the tool. Use the OnUpdate method(below) to set the enable\disable state of the tool.
//Add the OnUpdate method to the tool.
//Note: since OnUpdate is called very frequently, you should avoid lengthy operations in this method 
//as this would reduce the responsiveness of the application user interface.
internal class SnippetButton : ArcGIS.Desktop.Framework.Contracts.Button
{
  protected override void OnUpdate()
  {
    bool enableSate = true; //TODO: Code your enabled state  
    bool criteria = true;  //TODO: Evaluate criteria for disabledText  

    if (enableSate)
    {
      this.Enabled = true;  //tool is enabled  
    }
    else
    {
      this.Enabled = false;  //tool is disabled  
                             //customize your disabledText here  
      if (criteria)
        this.DisabledTooltip = "Missing criteria 1";
    }
  }
}