# EstimatorChain<TLastTransformer>EstimatorChain<TLastTransformer>EstimatorChain<TLastTransformer> Class

## Definition

Represents a chain (potentially empty) of estimators that end with a TLastTransformer. If the chain is empty, TLastTransformer is always ITransformer.

public sealed class EstimatorChain<TLastTransformer> : Microsoft.ML.IEstimator<Microsoft.ML.Data.TransformerChain<TLastTransformer>> where TLastTransformer : class, ITransformer
type EstimatorChain<'LastTransformer (requires 'LastTransformer : null and 'LastTransformer :> ITransformer)> = class
interface IEstimator<TransformerChain<'LastTransformer>>
Public NotInheritable Class EstimatorChain(Of TLastTransformer)
Implements IEstimator(Of TransformerChain(Of TLastTransformer))

#### Type Parameters

TLastTransformer
Inheritance
EstimatorChain<TLastTransformer>EstimatorChain<TLastTransformer>EstimatorChain<TLastTransformer>
Implements
IEstimator<TransformerChain<TLastTransformer>>

## Constructors

 EstimatorChain() EstimatorChain() EstimatorChain() Create an empty estimator chain.

## Fields

 LastEstimator LastEstimator LastEstimator

## Methods

 Append(IEstimator, TransformerScope) Append(IEstimator, TransformerScope) Append(IEstimator, TransformerScope) AppendCacheCheckpoint(IHostEnvironment) AppendCacheCheckpoint(IHostEnvironment) AppendCacheCheckpoint(IHostEnvironment) Append a 'caching checkpoint' to the estimator chain. This will ensure that the downstream estimators will be trained against cached data. It is helpful to have a caching checkpoint before trainers that take multiple data passes. Fit(IDataView) Fit(IDataView) Fit(IDataView) GetOutputSchema(SchemaShape) GetOutputSchema(SchemaShape) GetOutputSchema(SchemaShape)

## Extension Methods

 WithOnFitDelegate(IEstimator, Action) WithOnFitDelegate(IEstimator, Action) WithOnFitDelegate(IEstimator, Action) Given an estimator, return a wrapping object that will call a delegate once Fit(IDataView) is called. It is often important for an estimator to return information about what was fit, which is why the Fit(IDataView) method returns a specifically typed object, rather than just a general ITransformer. However, at the same time, IEstimator are often formed into pipelines with many objects, so we may need to build a chain of estimators via EstimatorChain where the estimator for which we want to get the transformer is buried somewhere in this chain. For that scenario, we can through this method attach a delegate that will be called once fit is called.