@@ -57,8 +57,8 @@ namespace NLog.LayoutRenderers
         /// <param name="logEvent">Logging event.</param>
         protected override void Append(StringBuilder builder, LogEventInfo logEvent)
         {
-            string msg = GlobalDiagnosticsContext.Get(this.Item);
-            builder.Append(msg);
+            // use AppendFormat to ensure that any IFormatProvider is respected
+            builder.AppendFormat(logEvent.FormatProvider, "{0}", GlobalDiagnosticsContext.GetObject(this.Item));
         }
     }
 }
// 
// Copyright (c) 2004-2011 Jaroslaw Kowalski <jaak@jkowalski.net>
// 
// All rights reserved.
// 
// Redistribution and use in source and binary forms, with or without 
// modification, are permitted provided that the following conditions 
// are met:
// 
// * Redistributions of source code must retain the above copyright notice, 
//   this list of conditions and the following disclaimer. 
// 
// * Redistributions in binary form must reproduce the above copyright notice,
//   this list of conditions and the following disclaimer in the documentation
//   and/or other materials provided with the distribution. 
// 
// * Neither the name of Jaroslaw Kowalski nor the names of its 
//   contributors may be used to endorse or promote products derived from this
//   software without specific prior written permission. 
// 
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE 
// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
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// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
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// THE POSSIBILITY OF SUCH DAMAGE.
// 

namespace NLog.LayoutRenderers
{
    using System.Text;
    using NLog.Config;

    /// <summary>
    /// Global Diagnostics Context item. Provided for compatibility with log4net.
    /// </summary>
    [LayoutRenderer("gdc")]
    public class GdcLayoutRenderer : LayoutRenderer
    {
        /// <summary>
        /// Gets or sets the name of the item.
        /// </summary>
        /// <docgen category='Rendering Options' order='10' />
        [RequiredParameter]
        [DefaultParameter]
        public string Item { get; set; }

        /// <summary>
        /// Renders the specified Global Diagnostics Context item and appends it to the specified <see cref="StringBuilder" />.
        /// </summary>
        /// <param name="builder">The <see cref="StringBuilder"/> to append the rendered data to.</param>
        /// <param name="logEvent">Logging event.</param>
        protected override void Append(StringBuilder builder, LogEventInfo logEvent)
        {
            string msg = GlobalDiagnosticsContext.Get(this.Item);
            builder.Append(msg);
        }
    }
}

This looks a bit strange. (string.format and "{0}") I propose: `GlobalDiagnosticsContext.Get(this.item, logEvent.FormatProvider)`. - (so a new overload)