Running a geoprocess inside C# (AO)

geoprocessor.cs

public void SampleCalculateBestPathToolGping()
{
    // Initialize the geoprocessor.
    IGeoProcessor2 gp=new GeoProcessorClass();

    // Add the BestPath toolbox.
    gp.AddToolbox(@"C:\SanDiego\BestPath.tbx");

    // Generate the array of parameters.
    IVariantArray parameters=new VarArrayClass();
    parameters.Add(@"C:\SanDiego\source.shp");
    parameters.Add(@"C:\SanDiego\destination.shp");
    parameters.Add(@"C:\SanDiego\bestpath.shp");

    object sev=null;
    try
    {
        // Execute the model tool by name.
        gp.Execute("CalculateBestPath", parameters, null);

        Console.WriteLine(gp.GetMessages(ref sev));
    }
    catch (Exception)
    {
        // Print geoprocessing execution error messages.
        Console.WriteLine(gp.GetMessages(ref sev));
        throw;
    }
}

http://desktop.arcgis.com/en/arcobjects/latest/net/webframe.htm#c4ff8b68-0410-435f-b8e5-682d5cea47cf.htm