非静态的字段，方法或属性要求对象引用如何解决？

写了一个公共类DBhelper.cs，但是在引用的时候出现““ 非静态的字段、方法或属性“DBhelper.ExecuteNonQuery(STRING,System.Data.SqlClient.SqlParameter[])”要求对象引用””，应该怎么样解决呢？

附DBhelper.cs代码如下：
public class DBhelper
{
private SqlConnection connection;
private SqlCommand command;
private SqlDataAdapter adapter;
public DBhelper()
{
this.connection = new SqlConnection(ConfigurationManager.ConnectionStrings["db_EShopConnectionString"].ConnectionString);
this.command = new SqlCommand();
this.command.Connection = this.connection;
this.adapter = new SqlDataAdapter(command);
}

public int ExecuteNonQuery(string sql)
{
return this.ExecuteNonQuery(sql, new SqlParameter[0]);
}

public int ExecuteNonQuery(string sql, SqlParameter[] param)
{
this.command.CommandText = sql;
this.command.Parameters.Clear();
for (int i = 0; i < param.Length; i++)
{
if (param[i] != null)
{
this.command.Parameters.Add(param[i]);

}
int result = 0;
try
{
this.connection.Open();
result = this.command.ExecuteNonQuery();
}
catch (Exception ex)
{
throw ex;
}
finally
{
this.connection.Close();
}
return result;
}
return 0;
}

public DataTable Select(string sql, SqlParameter[] param)
{
this.command.CommandText = sql;
this.command.Parameters.Clear();
for (int i = 0; i < param.Length; i++)
{
if (param[i] != null)
{
this.command.Parameters.Add(param[i]);

}
DataTable dtData = new DataTable();
try
{
this.adapter.Fill(dtData);
}
catch (Exception ex)
{
throw ex;
}
return dtData;
}
return null;
}

public DataTable Select(string sql)
{
return this.Select(sql, new SqlParameter[0]);
}

public static SqlParameter[] AddParameter(SqlParameter[] paramArray, SqlParameter param)
{
Array.Resize<SqlParameter>(ref paramArray, paramArray.Length + 1);
return paramArray;
}
}