public override bool IsContentTypeSupported(string contentType) 
{ 
    if (base.IsContentTypeSupported(contentType)) 
    { 
        return true; 
    } 
    if (contentType.Length == this.MediaType.Length) 
    { 
        return contentType.Equals(this.MediaType, StringComparison.OrdinalIgnoreCase); 
    } 
    else 
    { 
        if (contentType.StartsWith(this.MediaType, StringComparison.OrdinalIgnoreCase) 
            && (contentType[this.MediaType.Length] == ';')) 
        { 
            return true; 
        } 
    } 
    return false; 
}

public class CustomTextMessageEncoderFactory : MessageEncoderFactory
{
    private MessageEncoder encoder;
    private MessageVersion version;
    private string mediaType;
    private string charSet;

    internal CustomTextMessageEncoderFactory(string mediaType, string charSet,
        MessageVersion version)
    {
        this.version = version;
        this.mediaType = mediaType;
        this.charSet = charSet;
        this.encoder = new CustomTextMessageEncoder(this);
    }

    public override MessageEncoder Encoder
    {
        get 
        { 
            return this.encoder;
        }
    }

    public override MessageVersion MessageVersion
    {
        get 
        { 
            return this.version;
        }
    }

    internal string MediaType
    {
        get
        {
            return this.mediaType;
        }
    }

    internal string CharSet
    {
        get
        {
            return this.charSet;
        }
    }
}