#region Apache License
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//
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// Licensed to the Apache Software Foundation (ASF) under one or more
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// contributor license agreements. See the NOTICE file distributed with
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// this work for additional information regarding copyright ownership.
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// The ASF licenses this file to you under the Apache License, Version 2.0
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// (the "License"); you may not use this file except in compliance with
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// the License. You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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#endregion
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using System;
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using System.Collections;
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namespace log4net.Util
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{
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/// <summary>
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/// An always empty <see cref="IEnumerator"/>.
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/// </summary>
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/// <remarks>
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/// <para>
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/// A singleton implementation of the <see cref="IEnumerator"/> over a collection
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/// that is empty and not modifiable.
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/// </para>
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/// </remarks>
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/// <author>Nicko Cadell</author>
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/// <author>Gert Driesen</author>
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public sealed class NullEnumerator : IEnumerator
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{
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#region Private Instance Constructors
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/// <summary>
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/// Initializes a new instance of the <see cref="NullEnumerator" /> class.
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/// </summary>
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/// <remarks>
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/// <para>
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/// Uses a private access modifier to enforce the singleton pattern.
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/// </para>
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/// </remarks>
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private NullEnumerator()
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{
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}
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#endregion Private Instance Constructors
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#region Public Static Properties
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/// <summary>
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/// Get the singleton instance of the <see cref="NullEnumerator" />.
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/// </summary>
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/// <returns>The singleton instance of the <see cref="NullEnumerator" />.</returns>
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/// <remarks>
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/// <para>
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/// Gets the singleton instance of the <see cref="NullEnumerator" />.
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/// </para>
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/// </remarks>
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public static NullEnumerator Instance
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{
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get { return s_instance; }
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}
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#endregion Public Static Properties
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#region Implementation of IEnumerator
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/// <summary>
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/// Gets the current object from the enumerator.
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/// </summary>
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/// <remarks>
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/// Throws an <see cref="InvalidOperationException" /> because the
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/// <see cref="NullDictionaryEnumerator" /> never has a current value.
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/// </remarks>
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/// <remarks>
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/// <para>
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/// As the enumerator is over an empty collection its <see cref="Current"/>
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/// value cannot be moved over a valid position, therefore <see cref="Current"/>
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/// will throw an <see cref="InvalidOperationException"/>.
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/// </para>
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/// </remarks>
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/// <exception cref="InvalidOperationException">The collection is empty and <see cref="Current"/>
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/// cannot be positioned over a valid location.</exception>
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public object Current
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{
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get { throw new InvalidOperationException(); }
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}
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/// <summary>
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/// Test if the enumerator can advance, if so advance
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/// </summary>
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/// <returns><c>false</c> as the <see cref="NullEnumerator" /> cannot advance.</returns>
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/// <remarks>
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/// <para>
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/// As the enumerator is over an empty collection its <see cref="Current"/>
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/// value cannot be moved over a valid position, therefore <see cref="MoveNext"/>
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/// will always return <c>false</c>.
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/// </para>
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/// </remarks>
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public bool MoveNext()
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{
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return false;
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}
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/// <summary>
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/// Resets the enumerator back to the start.
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/// </summary>
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/// <remarks>
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/// <para>
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/// As the enumerator is over an empty collection <see cref="Reset"/> does nothing.
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/// </para>
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/// </remarks>
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public void Reset()
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{
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}
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#endregion Implementation of IEnumerator
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#region Private Static Fields
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/// <summary>
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/// The singleton instance of the <see cref="NullEnumerator" />.
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/// </summary>
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private readonly static NullEnumerator s_instance = new NullEnumerator();
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#endregion Private Static Fields
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}
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}
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