public T GetDefinition<T>( 
   string name
)
where T: Definition

public async Task ObtainingDefinitionFromGeodatabase()
{
    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
        using (Geodatabase geodatabase = new Geodatabase(new DatabaseConnectionFile(new Uri("path\\to\\sde\\file\\sdefile.sde"))))
        {
            // Remember that for Enterprise databases you have to qualify your dataset names with the DatabaseName and UserName.
            TableDefinition enterpriseTableDefinition = geodatabase.GetDefinition<TableDefinition>("LocalGovernment.GDB.CitizenContactInfo");

            // It does not matter if the dataset is within a FeatureDataset or not.
            FeatureClassDefinition featureClassDefinition = geodatabase.GetDefinition<FeatureClassDefinition>("LocalGovernment.GDB.FireStation");

            // GetDefinition For a RelationshipClass.
            RelationshipClassDefinition relationshipClassDefinition = geodatabase.GetDefinition<RelationshipClassDefinition>("LocalGovernment.GDB.AddressPointHasSiteAddresses");

            // GetDefinition For a FeatureDataset.
            FeatureDatasetDefinition featureDatasetDefinition = geodatabase.GetDefinition<FeatureDatasetDefinition>("LocalGovernment.GDB.Address");
        }
    });
}

public async Task ObtainingDefinitionsFromFS(string url)
{
    //Url examples for (federated) feature services
    //(by "ref" online):
    //https://sampleserver6.arcgisonline.com/arcgis/rest/services/LocalGovernment/Recreation/FeatureServer
    //(federated by ref on portal)
    //https://portal.example.com/server/rest/services/FeatureServiceName/FeatureServer
    //(federated by value - Hosted - on portal)
    //https://portal.example.com/server/rest/services/Hosted/FeatureServiceName/FeatureServer

    await QueuedTask.Run(() =>
    {
        var uri = new Uri(url, UriKind.Absolute);
        using (var fs_db = new ArcGIS.Core.Data.Geodatabase(new ServiceConnectionProperties(uri)))
        {
            //Assuming that id 0 is a FeatureClass for the given service
            FeatureClassDefinition featureClassDefinition =
                    fs_db.GetDefinition<FeatureClassDefinition>("0");
            string shapeField = featureClassDefinition.GetShapeField();
            IReadOnlyList<Field> fields = featureClassDefinition.GetFields();

            //Assuming that id 1 is a Table for the given service
            TableDefinition tableDefinition =
                        fs_db.GetDefinition<TableDefinition>("1");
            string objectIDField = tableDefinition.GetObjectIDField();
        }
    });
}

// Must be called within QueuedTask.Run9)
public bool TableExists(Geodatabase geodatabase, string tableName)
{
    try
    {
        TableDefinition tableDefinition = geodatabase.GetDefinition<TableDefinition>(tableName);
        tableDefinition.Dispose();
        return true;
    }
    catch
    {
        // GetDefinition throws an exception if the definition doesn't exist
        return false;
    }
}

// Must be called within QueuedTask.Run()
public bool FeatureClassExists(Geodatabase geodatabase, string featureClassName)
{
    try
    {
        FeatureClassDefinition featureClassDefinition = geodatabase.GetDefinition<FeatureClassDefinition>(featureClassName);
        featureClassDefinition.Dispose();
        return true;
    }
    catch
    {
        // GetDefinition throws an exception if the definition doesn't exist
        return false;
    }
}