IDataErrorInfo与Dictionary绑定

Question

我在View Model中实现了一个IDataErrorInfo。问题是验证词典的索引：

public class ViewModel : IDataErrorInfo
    {
        private Dictionary<string, string> reportFile;

public string this[string columnName]
        {
            get
            {
                string result = null;

                switch (columnName)
                {

如何验证每个Dictionary索引？