I have solved it by myself.
I just add code to it as follows:
public string FeatureClassToCoverage(string string_inFeatures, string string_outCoverage)
{
<big>ESRI.ArcGIS.esriSystem.IAoInitialize ao = new ESRI.ArcGIS.esriSystem.AoInitialize();
ao.Initialize(ESRI.ArcGIS.esriSystem.esriLicenseProductCode.esriLicenseProductCodeArcInfo);</big>
IWorkspaceFactory pWorkspaceFactory = new ShapefileWorkspaceFactory();
IWorkspace pWorkspace = pWorkspaceFactory.OpenFromFile(System.IO.Path.GetDirectoryName(string_inFeatures), 0);
IFeatureWorkspace pFeatureWorkspace = pWorkspace as IFeatureWorkspace;
IFeatureClass pFeatureClass = pFeatureWorkspace.OpenFeatureClass(System.IO.Path.GetFileNameWithoutExtension(string_inFeatures));
String outDict = System.IO.Path.GetDirectoryName(string_inFeatures);
String outCoverage = outDict + "\\" + string_outCoverage;
ESRI.ArcGIS.ConversionTools.FeatureclassToCoverage featureClassToCoverage = new FeatureclassToCoverage(pFeatureClass, outCoverage);
ESRI.ArcGIS.Geoprocessor.Geoprocessor gp = new Geoprocessor();
ESRI.ArcGIS.Geoprocessing.IGeoProcessorResult results = (ESRI.ArcGIS.Geoprocessing.IGeoProcessorResult)gp.Execute(featureClassToCoverage, null);
string returnString = outCoverage;
if (results.Status != ESRI.ArcGIS.esriSystem.esriJobStatus.esriJobSucceeded)
{
returnString = "Changing Failed!";
}
return returnString;
}