r@".NETFramework\v4.0\Profile\Client\System.Runtime.Serialization.dll"

open System.Runtime.Serialization
open System.Runtime.Serialization.Json

[<DataContract>]
    type geo = {
        [<field: DataMember(Name = "type")>]
        t:string
        [<field: DataMember(Name = "coordinates")>]
        coordinates:string
        }


let decode (s:string)  = 
    let json = new DataContractJsonSerializer(typeof<geo>)
    let byteArray = Encoding.UTF8.GetBytes(s)
    let stream = new MemoryStream(byteArray)
    json.ReadObject(stream) :?> geo

let tw = {"type":"Point","coordinates":[-7.002648,110.449961]}

decode tw 

This returns -> End element 'coordinates' from namespace '' expected. Found element 'item' from namespace ''

How can I define the DataMember coordinates so that it understands ?

Thanks a lot