Public Class MyX509CertificateValidator
    Inherits X509CertificateValidator
    Private allowedIssuerName As String

    Public Sub New(ByVal allowedIssuerName As String)
        If allowedIssuerName Is Nothing Then
            Throw New ArgumentNullException("allowedIssuerName")
        End If

        Me.allowedIssuerName = allowedIssuerName

    End Sub 'New

    Public Overrides Sub Validate(ByVal certificate As X509Certificate2)
        ' Check that there is a certificate.
        If certificate Is Nothing Then
            Throw New ArgumentNullException("certificate")
        End If

        ' Check that the certificate issuer matches the configured issuer
        If allowedIssuerName <> certificate.IssuerName.Name Then
            Throw New SecurityTokenValidationException("Certificate was not issued by a trusted issuer")
        End If

    End Sub 'Validate
End Class 'MyX509CertificateValidator

public class MyX509CertificateValidator : X509CertificateValidator
{
    string allowedIssuerName;
    public MyX509CertificateValidator(string allowedIssuerName)
    {
        if (allowedIssuerName == null)
        {
            throw new ArgumentNullException("allowedIssuerName");
        }

        this.allowedIssuerName = allowedIssuerName;
    }
    public override void Validate(X509Certificate2 certificate)
    {
        // Check that there is a certificate.
        if (certificate == null)
        {
            throw new ArgumentNullException("certificate");
        }

        // Check that the certificate issuer matches the configured issuer
        if (allowedIssuerName != certificate.IssuerName.Name)
        {
            throw new SecurityTokenValidationException
              ("Certificate was not issued by a trusted issuer");
        }
    }
}