Personally I would convert the text files to dBase or better still a GeoDatabase Table as then your data is in a Table which will make joining easier.
// Create a text file workspace factory. Type factoryType = Type.GetTypeFromProgID( "esriDataSourcesOleDB.TextFileWorkspaceFactory"); IWorkspaceFactory workspaceFactory = (IWorkspaceFactory)Activator.CreateInstance (factoryType); // Open a directory of CSV files using its path. IWorkspace workspace = workspaceFactory.OpenFromFile(@"C:\Data\CSV", 0); // Open a CSV file as a table. IFeatureWorkspace featureWorkspace = (IFeatureWorkspace)workspace; ITable table = featureWorkspace.OpenTable("Levee.csv");