Executing a system tool

// Add references to esriSystem for licensing and IVariantArray.
using ESRI.ArcGIS.esriSystem;
// Add a reference to the geoprocessing namespace.
using ESRI.ArcGIS.Geoprocessing;

private static void RunBuffer()
{
    // Create the geoprocessor.
    IGeoProcessor2 gp = new GeoProcessorClass();

    // Create an IVariantArray to hold the parameter values.
    IVariantArray parameters = new VarArrayClass();

    // Populate the variant array with parameter values.
    parameters.Add(@"D:\St_Johns\data.mdb\roads");
    parameters.Add(@"D:\St_Johns\data.mdb\roads_Buffer");
    parameters.Add("1000 Meters");

    // Execute the tool.
    gp.Execute("Buffer_analysis", parameters, null);
}