Dictionary<String, String> allTables = new Dictionary<String, String>();
allTables = tables1.Union(tables2).ToDictionary(pair => pair.Key, pair => pair.Value);

其平凡解为:

using System.Collections.Generic;
...
public static Dictionary<TKey, TValue>
    Merge<TKey,TValue>(IEnumerable<Dictionary<TKey, TValue>> dictionaries)
{
    var result = new Dictionary<TKey, TValue>();
    foreach (var dict in dictionaries)
        foreach (var x in dict)
            result[x.Key] = x.Value;
    return result;
}

我知道这是一个老问题，但是因为我们现在有LINQ，你可以像这样在一行中完成它

Dictionary<T1,T2> merged;
Dictionary<T1,T2> mergee;
mergee.ToList().ForEach(kvp => merged.Add(kvp.Key, kvp.Value));

or

mergee.ToList().ForEach(kvp => merged.Append(kvp));

using System.Collections.Generic;
using System.Linq;

public static class DictionaryExtensions
{
    public enum MergeKind { SkipDuplicates, OverwriteDuplicates }
    public static void Merge<K, V>(this IDictionary<K, V> target, IDictionary<K, V> source, MergeKind kind = MergeKind.SkipDuplicates) =>
        source.ToList().ForEach(_ => { if (kind == MergeKind.OverwriteDuplicates || !target.ContainsKey(_.Key)) target[_.Key] = _.Value; });
}

你可以跳过/忽略(默认)或覆盖副本:如果你对Linq性能不过分挑剔，而是像我一样喜欢简洁的可维护代码:在这种情况下，你可以删除默认的MergeKind。skipduplicate用于强制调用者进行选择，并使开发人员知道结果将是什么!

使用扩展方法合并。当存在重复的键时，它不会抛出异常，而是用第二个字典中的键替换这些键。

internal static class DictionaryExtensions
{
    public static Dictionary<T1, T2> Merge<T1, T2>(this Dictionary<T1, T2> first, Dictionary<T1, T2> second)
    {
        if (first == null) throw new ArgumentNullException("first");
        if (second == null) throw new ArgumentNullException("second");

        var merged = new Dictionary<T1, T2>();
        first.ToList().ForEach(kv => merged[kv.Key] = kv.Value);
        second.ToList().ForEach(kv => merged[kv.Key] = kv.Value);

        return merged;
    }
}

用法:

Dictionary<string, string> merged = first.Merge(second);

我将分解@orip的简单而非垃圾的创建解决方案，以提供除了Merge()之外的一个适当的AddAll()来处理将一个字典添加到另一个字典的简单情况。

using System.Collections.Generic;
...
public static Dictionary<TKey, TValue>
    AddAll<TKey,TValue>(Dictionary<TKey, TValue> dest, Dictionary<TKey, TValue> source)
{
    foreach (var x in source)
        dest[x.Key] = x.Value;
}

public static Dictionary<TKey, TValue>
    Merge<TKey,TValue>(IEnumerable<Dictionary<TKey, TValue>> dictionaries)
{
    var result = new Dictionary<TKey, TValue>();
    foreach (var dict in dictionaries)
        result.AddAll(dict);
    return result;
}