I want to set Geoprocessing environment "Output coordinatsystem" in ArcMap using ArcObjects code the same way that its possible menuitem "Geoprocessing/Environments..."

[OlavKavli]

To be able to set automaticly the Output Coordinatsystem to geoprocessing Environment to the same as current Dataframe.

[YuanLiu]

IGeoProcessor.SetEnvironmentValue sounds like what you need (http://resources.arcgis.com/en/help/arcobjects-net/componenthelp/index.html#//002n0000014r000000 )

Haven't tested it myself, but I think it should be some code like this: gp.SetEnvironmentValue("outputCoordinateSystem", theCoordiSysYouWant)