No.1 先定义一个SoapHeader
/// <summary>
/// 定义SoapHeader.
/// </summary>
public class CredentialSoapHeader : SoapHeader
{
    public string Username;
    public string PasswordHash;
    public string SecurityKey;
}
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No.2 定义安全类，继承WebService。并提供验证方法。
public class SecureWebService : WebService
{
    public CredentialSoapHeader Credentials;
    protected string VerifyCredentials()
    {
        if (this.Credentials == null
        || this.Credentials.Username == null
        || this.Credentials.PasswordHash == null)
        {
            throw new SoapException("没有提供信任令牌",
                SoapException.ClientFaultCode, "Security");
        }
        return CheckCredential(this.Credentials);
    }
}
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No.3 让你发布的webservice继承你的安全类。
public class SecureService : SecureWebService
{

    public SecureService()
    {

        //如果使用设计的组件，请取消注释以下行
        //InitializeComponent();
    }
}
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No.4发布你的Webservice。在客户端对SoapHeader赋值，调用Webservice。完成。