public class A
{
    static string rtnVal = string.Empty;//变量

    //可读写的属性P
    public static string P
    {
        get { return rtnVal; }
        set { rtnVal = value; }
    }
}

public class B
{
    //得到 A类中的P属性
    public void getAP()
    {
        Response.Write(A.P);
    }
}

------解决方案--------------------
C# code
 
 using System; 
 using System.Data; 
 using System.Configuration; 
 using System.Collections; 
 using System.Web; 
 using System.Web.Security; 
 using System.Web.UI; 
 using System.Web.UI.WebControls; 
 using System.Web.UI.WebControls.WebParts; 
 using System.Web.UI.HtmlControls; 
  
  
 public partial class Temp : System.Web.UI.Page 
 { 
   protected void Page_Load(object sender, EventArgs e) 
   { 
     string tempStr=tempClass.tempStaticStr;//tempClass.tempStaticStr 
     tempStr=new tempClass().tempStr;//TempClass.tempStr 
     tempStr = namespaceName.hasNamespaceClass.tempStaticStr;//namespaceName.hasNamespaceClass.tempStaticStr 
     tempStr = new namespaceName.hasNamespaceClass().tempStr;//namespaceName.hasNamespaceClass.tempStr 
      
   } 
 } 
  
  
 tempClass 在App_Code下 无命名空间 
  
 using System; 
 using System.Data; 
 using System.Configuration; 
 using System.Web; 
 using System.Web.Security; 
 using System.Web.UI; 
 using System.Web.UI.WebControls; 
 using System.Web.UI.WebControls.WebParts; 
 using System.Web.UI.HtmlControls; 
  
 ///  <summary> 
 /// Temp 的摘要说明 
 ///  </summary> 
 public class tempClass 
 { 
   public tempClass() 
 	{ 
 		// 
 		// TODO: 在此处添加构造函数逻辑 
 		// 
 	} 
   public static string tempStaticStr = "tempClass.tempStaticStr"; 
   public string tempStr = "tempClass.tempStr"; 
 } 
  
  
 hasNamespaceClass在App_Code下 有命名空间 命名空间名为namespaceName 
 using System; 
 using System.Data; 
 using System.Configuration; 
 using System.Web; 
 using System.Web.Security; 
 using System.Web.UI; 
 using System.Web.UI.WebControls; 
 using System.Web.UI.WebControls.WebParts; 
 using System.Web.UI.HtmlControls; 
  
  
 namespace namespaceName 
 { 
   ///  <summary> 
   /// hasNamespaceClass 的摘要说明 
   ///  </summary> 
   public class hasNamespaceClass 
   { 
     public hasNamespaceClass() 
     { 
       // 
       // TODO: 在此处添加构造函数逻辑 
       // 
     } 
     public static string tempStaticStr = "namespaceName.hasNamespaceClass.tempStaticStr"; 
     public string tempStr = "namespaceName.hasNamespaceClass.tempStr";