The C#-collection has been modified; enumeration operations may not be performed

Today, when I use the foreach statement to iterate over the dictionary, I find an error:

The collection has been modified; enumeration operations may not be performed

Search the web, and find that foreach is iterative, and it cannot be added and deleted. But, I tried, and found that there was another problem: when comparing and correcting values, it also gives an error, such as:

1 foreach(keyvaluepair<string,Double> soilstrkvinchsoil)2             {3                 if(Isvalidval (soilstrkv.value))4                 {5Soil[soilstrkv.key] =Replaceinvalidval;6                     //sitenumstr = soilStrKV.Key.Substring (0, SoilStrKV.Key.IndexOf ('_'));7                     //Yearstr8                     //Sitesoil.add ();9                 }Ten             } One  A /// <summary> -         ///False indicates a non-conforming value, otherwise conforming. -         /// </summary> the         /// <param name= "inval" ></param> -         /// <returns></returns> -         Private BOOLIsvalidval (Doubleinval) -         { +             returnInval <=Available_val; -}

In line 5th, the error is corrected (when the duplicate value is modified).

The C#-collection has been modified; enumeration operations may not be performed