X509Extension Class

Represents an X509 extension.

Namespace:   System.Security.Cryptography.X509Certificates
Assembly:  System (in System.dll)

public class X509Extension : AsnEncodedData
public ref class X509Extension : AsnEncodedData
type X509Extension = 
        inherit AsnEncodedData
Public Class X509Extension
	Inherits AsnEncodedData
System_CAPS_protmethod X509Extension

Initializes a new instance of the X509Extension class.

System_CAPS_pubmethod X509Extension

Initializes a new instance of the X509Extension class.

System_CAPS_pubmethod X509Extension

Initializes a new instance of the X509Extension class.

System_CAPS_pubmethod X509Extension

Initializes a new instance of the X509Extension class.


Gets a Boolean value indicating whether the extension is critical.


Gets or sets the Oid value for an AsnEncodedData object.(Inherited from AsnEncodedData.)


Gets or sets the Abstract Syntax Notation One (ASN.1)-encoded data represented in a byte array.(Inherited from AsnEncodedData.)

System_CAPS_pubmethod CopyFrom

Copies the extension properties of the specified AsnEncodedData object.(Overrides AsnEncodedDataCopyFrom.)

System_CAPS_pubmethod Equals

Determines whether the specified object is equal to the current object.(Inherited from Object.)

System_CAPS_protmethod Finalize

Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)

System_CAPS_pubmethod Format

Returns a formatted version of the Abstract Syntax Notation One (ASN.1)-encoded data as a string.(Inherited from AsnEncodedData.)

System_CAPS_pubmethod GetHashCode

Serves as the default hash function. (Inherited from Object.)

System_CAPS_pubmethod GetType

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_protmethod MemberwiseClone

Creates a shallow copy of the current Object.(Inherited from Object.)

System_CAPS_pubmethod ToString

Returns a string that represents the current object.(Inherited from Object.)


X509 extensions are dynamic, extended properties that can be added to an X509 certificate and changed. The X509Extension class can be used to create extensions that are associated with a certificate but are not part of a certificate as issued by a certification authority (CA).

In its most basic form, an X509 extension has an object identifier (OID), a Boolean value describing whether the extension is considered critical or not, and ASN-encoded data. Custom extensions can be registered in a CryptoConfig file.

The.NET Framework includes implementations of several common X509 extensions:


The following code example demonstrates using the X509Extension class.

using System;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;

public class CertSelect
    public static void Main()
            X509Store store = new X509Store("MY", StoreLocation.CurrentUser);
            store.Open(OpenFlags.ReadOnly | OpenFlags.OpenExistingOnly);

            X509Certificate2Collection collection = (X509Certificate2Collection)store.Certificates;
            for (int i = 0; i < collection.Count; i++)
                foreach (X509Extension extension in collection[i].Extensions)
                    Console.WriteLine(extension.Oid.FriendlyName + "(" + extension.Oid.Value + ")");

                    if (extension.Oid.FriendlyName == "Key Usage")
                        X509KeyUsageExtension ext = (X509KeyUsageExtension)extension;

                    if (extension.Oid.FriendlyName == "Basic Constraints")
                        X509BasicConstraintsExtension ext = (X509BasicConstraintsExtension)extension;

                    if (extension.Oid.FriendlyName == "Subject Key Identifier")
                        X509SubjectKeyIdentifierExtension ext = (X509SubjectKeyIdentifierExtension)extension;

                    if (extension.Oid.FriendlyName == "Enhanced Key Usage")
                        X509EnhancedKeyUsageExtension ext = (X509EnhancedKeyUsageExtension)extension;
                        OidCollection oids = ext.EnhancedKeyUsages;
                        foreach (Oid oid in oids)
                            Console.WriteLine(oid.FriendlyName + "(" + oid.Value + ")");
        catch (CryptographicException)
            Console.WriteLine("Information could not be written out for this certificate.");
Imports System
Imports System.Security.Cryptography
Imports System.Security.Cryptography.X509Certificates

Module CertSelect

    Sub Main()
            Dim store As New X509Store("MY", StoreLocation.CurrentUser)
            store.Open(OpenFlags.ReadOnly Or OpenFlags.OpenExistingOnly)

            Dim collection As X509Certificate2Collection = CType(store.Certificates, X509Certificate2Collection)
            Dim i As Integer
            For i = 0 To collection.Count
                Dim extension As X509Extension
                For Each extension In collection(i).Extensions
                    Console.WriteLine(extension.Oid.FriendlyName + "(" + extension.Oid.Value + ")")

                    If extension.Oid.FriendlyName = "Key Usage" Then
                        Dim ext As X509KeyUsageExtension = CType(extension, X509KeyUsageExtension)
                    End If

                    If extension.Oid.FriendlyName = "Basic Constraints" Then
                        Dim ext As X509BasicConstraintsExtension = CType(extension, X509BasicConstraintsExtension)
                    End If

                    If extension.Oid.FriendlyName = "Subject Key Identifier" Then
                        Dim ext As X509SubjectKeyIdentifierExtension = CType(extension, X509SubjectKeyIdentifierExtension)
                    End If

                    If extension.Oid.FriendlyName = "Enhanced Key Usage" Then
                        Dim ext As X509EnhancedKeyUsageExtension = CType(extension, X509EnhancedKeyUsageExtension)
                        Dim oids As OidCollection = ext.EnhancedKeyUsages
                        Dim oid As Oid
                        For Each oid In oids
                            Console.WriteLine(oid.FriendlyName + "(" + oid.Value + ")")
                        Next oid
                    End If
                Next extension
            Next i
            Console.WriteLine("Information could not be written out for this certificate.")
        End Try

    End Sub
End Module
#using <System.dll>
#using <system.security.dll>

using namespace System;
using namespace System::Security::Cryptography;
using namespace System::Security::Cryptography::X509Certificates;
int main()
      X509Store^ store = gcnew X509Store( L"MY",StoreLocation::CurrentUser );
      store->Open( static_cast<OpenFlags>(OpenFlags::ReadOnly | OpenFlags::OpenExistingOnly) );
      X509Certificate2Collection^ collection = dynamic_cast<X509Certificate2Collection^>(store->Certificates);
      for ( int i = 0; i < collection->Count; i++ )
         System::Collections::IEnumerator^ myEnum = collection[ i ]->Extensions->GetEnumerator();
         while ( myEnum->MoveNext() )
            X509Extension^ extension = safe_cast<X509Extension^>(myEnum->Current);
            Console::WriteLine( L"{0}({1})", extension->Oid->FriendlyName, extension->Oid->Value );
            if ( extension->Oid->FriendlyName == L"Key Usage" )
               X509KeyUsageExtension^ ext = dynamic_cast<X509KeyUsageExtension^>(extension);
               Console::WriteLine( ext->KeyUsages );
            if ( extension->Oid->FriendlyName == L"Basic Constraints" )
               X509BasicConstraintsExtension^ ext = dynamic_cast<X509BasicConstraintsExtension^>(extension);
               Console::WriteLine( ext->CertificateAuthority );
               Console::WriteLine( ext->HasPathLengthConstraint );
               Console::WriteLine( ext->PathLengthConstraint );
            if ( extension->Oid->FriendlyName == L"Subject Key Identifier" )
               X509SubjectKeyIdentifierExtension^ ext = dynamic_cast<X509SubjectKeyIdentifierExtension^>(extension);
               Console::WriteLine( ext->SubjectKeyIdentifier );
            if ( extension->Oid->FriendlyName == L"Enhanced Key Usage" )
               X509EnhancedKeyUsageExtension^ ext = dynamic_cast<X509EnhancedKeyUsageExtension^>(extension);
               OidCollection^ oids = ext->EnhancedKeyUsages;
               System::Collections::IEnumerator^ myEnum1 = oids->GetEnumerator();
               while ( myEnum1->MoveNext() )
                  Oid^ oid = safe_cast<Oid^>(myEnum1->Current);
                  Console::WriteLine( L"{0}({1})", oid->FriendlyName, oid->Value );

   catch ( CryptographicException^ ) 
      Console::WriteLine( L"Information could not be written out for this certificate." );

Version Information
Universal Windows Platform
Available since 10
.NET Framework
Available since 2.0
Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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