You need to use the below namespaces.
Microsoft.Azure.Management.ResourceManager.Fluent Microsoft.Azure.Management.Compute.Fluent.Models
var Auth_credentials = SdkContext.AzureCredentialsFactory.FromFile(@"C:\Tools\azureauth.properties");
var az = Azure
.Configure()
.WithLogLevel(HttpLoggingDelegatingHandler.Level.Basic)
.Authenticate(Auth_credentials)
.WithDefaultSubscription();
var virtualMachine = az.VirtualMachines.GetByResourceGroup("resourceGroupName", "vmName");
Console.WriteLine(virtualMachine.Name);
DataDisk disc = new DataDisk(virtualMachine.StorageProfile.DataDisks.Count + 1, DiskCreateOptionTypes.Empty, null, 15, null);
disc.Validate();
virtualMachine.StorageProfile.DataDisks.Add(disc);
virtualMachine.StorageProfile.Validate();
virtualMachine.Update().Apply();

For further information, refer to the SO link1 and SO link2.