IMap.AddLayer Method (ArcObjects .NET 10.6 SDK)
ArcObjects Library Reference (Carto)  

IMap.AddLayer Method

Adds a layer to the map.

[Visual Basic .NET]
Public Sub AddLayer ( _
    ByVal Layer As ILayer _
)
[C#]
public void AddLayer (
    ILayer Layer
);
[C++]
HRESULT AddLayer(
  ILayer* Layer
);
[C++]

Parameters Layer [in]
Layer is a parameter of type ILayer*

Product Availability

Available with ArcGIS Engine, ArcGIS Desktop, and ArcGIS Server.

Remarks

The AddLayer method adds a layer to the Map. Use the LayerCount property to get the total number of layers in the map.
AddLayer automatically attempts to set the Map's SpatialReference property if a coordinate system has not yet been defined for the map.  When the SpatialReference property is set, the Map's MapUnits and DistanceUnits properties are additionally set.  AddLayer also sets the spatial reference of the layer (ILayer::SpatialReference ). 
If no layers have a spatial reference, AddLayer checks the extent of the first layer (ILayer::AreaOfInterest) and if it has coordinates that look like geographic coordinates (XMin >= -180 and XMax <= 180 and YMin >= -90 and YMax <= 90), ArcMap assumes the data is in decimal degrees and sets the MapUnits to esriDecimalDegrees and DistanceUnits to esriMiles.
If no spatial reference is found and the coordinates do not look like geographic coordinates, ArcMap sets no spatial reference and sets the MapUnits to esriMeters and the DistanceUnits to esriMeters.
The full extent is recalculated each time a layer added.

See Also

IMap Interface | IMap.LayerCount Property | IMap.AddLayers Method

.NET Snippets

Add Layer File to ActiveView | Add GeoDataset Raster To Map | Add Shapefile Using OpenFileDialog | Add Tracking Layer To Map

.NET Related Topics

Creating and working with basemap layers | How to add a layer to a map from a layer file | How to add different types of layers to a map | How to create a mosaic layer | How to create a raster catalog layer | How to create a raster layer