public class MyClass : ITransientDependency
{
  public IAbpSession AbpSession { get; set; }

  public MyClass()
  {
    AbpSession = NullAbpSession.Instance;
  }

  public void MyMethod()
  {
    var currentUserId = AbpSession.UserId;
    //...
  }
}

using Abp.MultiTenancy;

namespace Abp.Runtime.Session
{
  public interface IAbpSession
  {
    long? UserId { get; }
    int? TenantId { get; }
    MultiTenancySides MultiTenancySide { get; }
    long? ImpersonatorUserId { get; }
    int? ImpersonatorTenantId { get; }
  }
}

using Abp.MultiTenancy;

namespace Abp.Runtime.Session
{
  /// <summary>
  /// Implements null object pattern for <see cref="IAbpSession"/>.
  /// </summary>
  public class NullAbpSession : IAbpSession
  {
    /// <summary>
    /// Singleton instance.
    /// </summary>
    public static NullAbpSession Instance { get { return SingletonInstance; } }
    private static readonly NullAbpSession SingletonInstance = new NullAbpSession();

    /// <inheritdoc/>
    public long? UserId { get { return null; } }

    /// <inheritdoc/>
    public int? TenantId { get { return null; } }

    public MultiTenancySides MultiTenancySide { get { return MultiTenancySides.Tenant; } }
    
    public long? ImpersonatorUserId { get { return null; } }
    
    public int? ImpersonatorTenantId { get { return null; } }

    private NullAbpSession()
    {

    }
  }
}

using System;
using System.Linq;
using System.Security.Claims;
using System.Threading;
using Abp.Configuration.Startup;
using Abp.MultiTenancy;
using Abp.Runtime.Security;

namespace Abp.Runtime.Session
{
  /// <summary>
  /// Implements <see cref="IAbpSession"/> to get session properties from claims of <see cref="Thread.CurrentPrincipal"/>.
  /// </summary>
  public class ClaimsAbpSession : IAbpSession
  {
    private const int DefaultTenantId = 1;

    public virtual long? UserId
    {
      get
      {
        var claimsPrincipal = Thread.CurrentPrincipal as ClaimsPrincipal;
        if (claimsPrincipal == null)
        {
          return null;
        }

        var claimsIdentity = claimsPrincipal.Identity as ClaimsIdentity;
        if (claimsIdentity == null)
        {
          return null;
        }

        var userIdClaim = claimsIdentity.Claims.FirstOrDefault(c => c.Type == ClaimTypes.NameIdentifier);
        if (userIdClaim == null || string.IsNullOrEmpty(userIdClaim.Value))
        {
          return null;
        }

        long userId;
        if (!long.TryParse(userIdClaim.Value, out userId))
        {
          return null;
        }

        return userId;
      }
    }

    public virtual int? TenantId
    {
      get
      {
        if (!_multiTenancy.IsEnabled)
        {
          return DefaultTenantId;
        }

        var claimsPrincipal = Thread.CurrentPrincipal as ClaimsPrincipal;
        if (claimsPrincipal == null)
        {
          return null;
        }

        var tenantIdClaim = claimsPrincipal.Claims.FirstOrDefault(c => c.Type == AbpClaimTypes.TenantId);
        if (tenantIdClaim == null || string.IsNullOrEmpty(tenantIdClaim.Value))
        {
          return null;
        }

        return Convert.ToInt32(tenantIdClaim.Value);
      }
    }

    public virtual MultiTenancySides MultiTenancySide
    {
      get
      {
        return _multiTenancy.IsEnabled && !TenantId.HasValue
          ? MultiTenancySides.Host
          : MultiTenancySides.Tenant;
      }
    }

    public virtual long? ImpersonatorUserId
    {
      get
      {
        var claimsPrincipal = Thread.CurrentPrincipal as ClaimsPrincipal;
        if (claimsPrincipal == null)
        {
          return null;
        }

        var impersonatorUserIdClaim = claimsPrincipal.Claims.FirstOrDefault(c => c.Type == AbpClaimTypes.ImpersonatorUserId);
        if (impersonatorUserIdClaim == null || string.IsNullOrEmpty(impersonatorUserIdClaim.Value))
        {
          return null;
        }

        return Convert.ToInt64(impersonatorUserIdClaim.Value);
      }
    }

    public virtual int? ImpersonatorTenantId
    {
      get
      {
        if (!_multiTenancy.IsEnabled)
        {
          return DefaultTenantId;
        }

        var claimsPrincipal = Thread.CurrentPrincipal as ClaimsPrincipal;
        if (claimsPrincipal == null)
        {
          return null;
        }

        var impersonatorTenantIdClaim = claimsPrincipal.Claims.FirstOrDefault(c => c.Type == AbpClaimTypes.ImpersonatorTenantId);
        if (impersonatorTenantIdClaim == null || string.IsNullOrEmpty(impersonatorTenantIdClaim.Value))
        {
          return null;
        }

        return Convert.ToInt32(impersonatorTenantIdClaim.Value);
      }
    }

    private readonly IMultiTenancyConfig _multiTenancy;

    /// <summary>
    /// Constructor.
    /// </summary>
    public ClaimsAbpSession(IMultiTenancyConfig multiTenancy)
    {
      _multiTenancy = multiTenancy;
    }
  }
}

using System;

namespace Abp.Runtime.Session
{
  /// <summary>
  /// Extension methods for <see cref="IAbpSession"/>.
  /// </summary>
  public static class AbpSessionExtensions
  {
    /// <summary>
    /// Gets current User's Id.
    /// Throws <see cref="AbpException"/> if <see cref="IAbpSession.UserId"/> is null.
    /// </summary>
    /// <param name="session">Session object.</param>
    /// <returns>Current User's Id.</returns>
    public static long GetUserId(this IAbpSession session)
    {
      if (!session.UserId.HasValue)
      {
        throw new AbpException("Session.UserId is null! Probably, user is not logged in.");
      }

      return session.UserId.Value;
    }

    /// <summary>
    /// Gets current Tenant's Id.
    /// Throws <see cref="AbpException"/> if <see cref="IAbpSession.TenantId"/> is null.
    /// </summary>
    /// <param name="session">Session object.</param>
    /// <returns>Current Tenant's Id.</returns>
    /// <exception cref="AbpException"></exception>
    public static int GetTenantId(this IAbpSession session)
    {
      if (!session.TenantId.HasValue)
      {
        throw new AbpException("Session.TenantId is null! Possible problems: No user logged in or current logged in user in a host user (TenantId is always null for host users).");
      }

      return session.TenantId.Value;
    }

    /// <summary>
    /// Creates <see cref="UserIdentifier"/> from given session.
    /// Returns null if <see cref="IAbpSession.UserId"/> is null.
    /// </summary>
    /// <param name="session">The session.</param>
    public static UserIdentifier ToUserIdentifier(this IAbpSession session)
    {
      return session.UserId == null
        ? null
        : new UserIdentifier(session.TenantId, session.GetUserId());
    }
  }
}