private void PrintPrimeMeridian()
    {
        // use activator class with SpatialReferenceEnvironment singleton
        Type factoryType = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
        System.Object obj = Activator.CreateInstance(factoryType);
        ISpatialReferenceFactory spatialReferenceFactory = obj as ISpatialReferenceFactory;
        ISet primeMeridiansSet = spatialReferenceFactory.CreatePredefinedPrimeMeridians();
        System.Windows.Forms.MessageBox.Show("Number of projections = " + primeMeridiansSet.Count);
        for (int i = 0; i < primeMeridiansSet.Count; i++)
        {
            IPrimeMeridian primeMeridian = primeMeridiansSet.Next() as IPrimeMeridian;
            System.Windows.Forms.MessageBox.Show(primeMeridian.Name);
        }
    }

//This example shows how the CreatePredefinedProjections function returns a 
//set that contains all the available Projection objects. The set is iterated 
//through, and the name of each Projection with the set is obtained. These  
//type of functions are useful for developers who may wish to populate a 
//pulldown selection list of available SpatialReference objects. 
private void PrintProjections()
{
  ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
  ISet projectionSet = spatialReferenceFactory.CreatePredefinedProjections();
  System.Windows.Forms.MessageBox.Show("Number of projections = " + projectionSet.Count);
  for (int i = 0; i < projectionSet.Count; i++)
  {
    IProjection projection = projectionSet.Next() as IProjection;
    System.Windows.Forms.MessageBox.Show(projection.Name);
  }
}

public IPrimeMeridian CreatePrimeMeridian (int primeMeridianType);

 ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
//Create the prime meridian using the available prime meridians. These can be
//found in the esriGeometry esriSRPrimeMType enumeration.
IPrimeMeridian primeMeridian = spatialReferenceFactory.CreatePrimeMeridian((int)esriSRPrimeMType.esriSRPrimeM_Greenwich);

public IProjection CreateProjection (int projectionType);

            ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
            //Create a projections using the available projections. These can be
            //found in the esriGeometry esriSRProjectionType, esriSRProjection2Type,
            //esriSRProjection3Type, and esriSRProjection4Type enumerations.
            IProjection projection = spatialReferenceFactory.CreateProjection((int)esriSRProjectionType.esriSRProjection_LambertConformalConic);

            ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
            //Create the spheroid using the available spheroids. These can be
            //found in the esriGeometry esriSRSpheroidType enumeration.
            ISpheroid spheroid = spatialReferenceFactory.CreateSpheroid((int)esriSRSpheroidType.esriSRSpheroid_Clarke1866);

Use an element from the esriSRUnitType or esriSRUnit2Type enumerations as the unitType to create a particular predefined unit of measure.
ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
//Create the unit using the available units.  These can be
//found in the esriGeometry esriSRUnitType enumeration.
ILinearUnit linearUnit = spatialReferenceFactory.CreateUnit((int)esriSRUnitType.esriSRUnit_Foot) as ILinearUnit;

public void ExportESRISpatialReferenceToPRJFile (
    stringprjFile,
    ISpatialReferenceSpatialReference);

public void ExportESRISpatialReferenceToPRJFileExample()
    {
        //The ISpatialReferenceFactory::ExportESRISpatialReferenceToPRJFile
        //method requires that you specify the path and filename of the output
        //PRJ file you wish to create with the export. A valid spatial reference
        //object containing spatial reference information is also required.
        // use activator class with SpatialReferenceEnvironment singleton
        Type factoryType = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
        System.Object obj = Activator.CreateInstance(factoryType);
        ISpatialReferenceFactory3 spatialReferenceFactory = obj as ISpatialReferenceFactory3;
        IProjectedCoordinateSystem projectedCoordinateSystem = spatialReferenceFactory.CreateProjectedCoordinateSystem((int)esriSRProjCSType.esriSRProjCS_WGS1984UTM_10N);
        //Export the pcs to a prj file
        String fileName = "c:\\temp\\utm10.prj";
        spatialReferenceFactory.ExportESRISpatialReferenceToPRJFile(fileName, projectedCoordinateSystem);
    }

private void CreateProjectedCoordinateSystem()
        {
            ISpatialReferenceFactory spatialReferenceFactory = new SpatialReferenceEnvironmentClass();
            //Create a projected coordinate system using the available projected coordinate systems
            IProjectedCoordinateSystem projectedCoordinateSystem1 = spatialReferenceFactory.CreateProjectedCoordinateSystem((int)esriSRProjCSType.esriSRProjCS_World_Mercator);
            //Here is an more detailed example for creating a pre-defined projected coordinate system for a new Shapefile using the 'ISpatialReferenceFactory::CreateProjectedCoordinateSystem method
            //Create the pre-defined projected coordinate system object
            IProjectedCoordinateSystem projectedCoordinateSystem2 = spatialReferenceFactory.CreateProjectedCoordinateSystem((int)esriSRProjCSType.esriSRProjCS_NAD1983SPCS_TXSouthCentFT);
            ISpatialReference spatialReference = projectedCoordinateSystem2 as ISpatialReference;
            //Set the false origin and units for the spatial reference.
            //You can use either the Domain or the FalseOriginAndUnits methods.
            // spatialReference.SetFalseOriginAndUnits(0, 0, 0);
            spatialReference.SetDomain(-1000000, 10000000, -1000000, 10000000);
            // spatialReference.SetMDomain(0, 1);
            spatialReference.SetMFalseOriginAndUnits(1, 1);
            // spatialReference.SetZDomain(0, 1);
            spatialReference.SetZFalseOriginAndUnits(1, 1);
            // Create a new SDE workspace
            IWorkspaceFactory sdeWorkspaceFactory = new ESRI.ArcGIS.DataSourcesGDB.SdeWorkspaceFactoryClass();
            IWorkspace workspace = sdeWorkspaceFactory.OpenFromFile("C:\\Documents and Settings\\bast5010\\Application Data\\ESRI\\ArcCatalog\\Connection to Vampire.sde", 0);
            IFeatureWorkspace sdeFeatureWorkspace = workspace as IFeatureWorkspace;
            //Create the fields for the feature dataset
            IFields fields = new FieldsClass();
            IFieldsEdit fieldsEdit = fields as IFieldsEdit;
            IField field = new FieldClass();
            IFieldEdit fieldEdit = field as IFieldEdit;
            fieldEdit.Type_2 = esriFieldType.esriFieldTypeOID;
            fieldEdit.IsNullable_2 = false;
            fieldEdit.Name_2 = "OID";
            fieldsEdit.AddField(fieldEdit);
            IGeometryDefEdit geometryDefEdit = new GeometryDefClass();
            geometryDefEdit.GeometryType_2 = esriGeometryType.esriGeometryPolygon;
            geometryDefEdit.GridCount_2 = 1;
            geometryDefEdit.set_GridSize(0, 1000);
            geometryDefEdit.SpatialReference_2 = spatialReference;
            fieldEdit = new FieldClass();
            fieldEdit.Name_2 = "Shape";
            fieldEdit.IsNullable_2 = true;
            fieldEdit.Type_2 = esriFieldType.esriFieldTypeGeometry;
            fieldEdit.GeometryDef_2 = geometryDefEdit;
            fieldsEdit.AddField(fieldEdit);
            fieldEdit = new FieldClass();
            fieldEdit.Name_2 = "Test_Field";
            fieldEdit.IsNullable_2 = true;
            fieldEdit.Editable_2 = true;
            fieldEdit.Length_2 = 25;
            fieldEdit.Type_2 = esriFieldType.esriFieldTypeString;
            fieldsEdit.AddField(fieldEdit);
            //Create a UID for the CreateFeatureClass method
            UID uid = new UIDClass();
            uid.Generate();
            //Create ExtCLSID as for CreateFeatureClass method
            UID uidExt = new UIDClass();
            uidExt.Generate();
            //Create the feature class for the feature dataset
            IFeatureClass featureClass = sdeFeatureWorkspace.CreateFeatureClass("PreDef_StateNAD83", fields, null, null, esriFeatureType.esriFTSimple, "Shape", "");
            System.Windows.Forms.MessageBox.Show("Data Creation Complete", "Program Status");
        }

private void CreateProjectedCoordinateSystem()
{
    // Set up the SpatialReferenceEnvironment.
    // SpatialReferenceEnvironment is a singleton object and needs to use the Activator class.
    Type t = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
    System.Object obj = Activator.CreateInstance(t);
    ISpatialReferenceFactory srFact = obj as ISpatialReferenceFactory;
    // Use the enumeration to create an instance of the predefined object.
    IProjectedCoordinateSystem projectedCS = srFact.CreateProjectedCoordinateSystem(
        (int)esriSRProjCSType.esriSRProjCS_NAD1983UTM_11N);
}

private IProjectedCoordinateSystem CreateProjectedCoordinateSystem()
{
    // Set up the SpatialReferenceEnvironment.
    // SpatialReferenceEnvironment is a singleton object and needs to use the Activator class.
    Type factoryType = Type.GetTypeFromProgID(
        "esriGeometry.SpatialReferenceEnvironment");
    System.Object obj = Activator.CreateInstance(factoryType);
    ISpatialReferenceFactory3 spatialReferenceFactory = obj as
        ISpatialReferenceFactory3;
    // Create a projection, GeographicCoordinateSystem, and unit using the factory.
    IProjectionGEN projection = spatialReferenceFactory.CreateProjection((int)
        esriSRProjectionType.esriSRProjection_Sinusoidal)as IProjectionGEN;
    IGeographicCoordinateSystem geographicCoordinateSystem =
        spatialReferenceFactory.CreateGeographicCoordinateSystem((int)
        esriSRGeoCSType.esriSRGeoCS_WGS1984);
    ILinearUnit unit = spatialReferenceFactory.CreateUnit((int)
        esriSRUnitType.esriSRUnit_Meter)as ILinearUnit;
    // Get the default parameters from the projection.
    IParameter[] parameters = projection.GetDefaultParameters();
    // Create a PCS using the Define method.
    IProjectedCoordinateSystemEdit projectedCoordinateSystemEdit = new
        ProjectedCoordinateSystemClass();
    object name = "Newfoundland";
    object alias = "NF_LAB";
    object abbreviation = "NF";
    object remarks = "Most Eastern Province in Canada";
    object usage = "When making maps of Newfoundland";
    object geographicCoordinateSystemObject = geographicCoordinateSystem as object;
    object unitObject = unit as object;
    object projectionObject = projection as object;
    object parametersObject = parameters as object;
    projectedCoordinateSystemEdit.Define(ref name, ref alias, ref abbreviation, ref
        remarks, ref usage, ref geographicCoordinateSystemObject, ref unitObject,
        ref projectionObject, ref parametersObject);
    return projectedCoordinateSystemEdit as IProjectedCoordinateSystem;
}