How to create a connection string in asp.net?

Are these related?

I will assume not...

Connection string:
1) In VS, open the Server Explorer (Menu: "View...Server Explorer"
2) Right click the database your are interested in, and select "Properties"
3) The connection string is listed in the properties window.
4) Open your Web.Config and below the "appSettings" section add:

<connectionStrings>
  <add name="MyDatabase" connectionString="...connection string as per the properties window"/>
</connectionStrings>

Objects and methods:
This is a bigger subject. I assume you have created your class library, and want to use items from them.
1) If you haven't done so, add a reference to the class library in you app.
2) Add a using if you want it.
3) Declare an object of the appropriate class type from your library, as if it was a .NET inbuilt type. for example if your class library defines a class "MyClass" then:

MyClass myInstanceOfMyClass = new MyClass();
myInstanceOfMyClass.MYMethod();

how do i create connection string in asp.net?

Whether it be ASP.NET or Winforms, forming connection string is same in all.

Look here for it: Connection String[^]

In class library how to create object and methode,how do i call them?

Create class. Objects are instances of the class. Methods are behaviours exposed in a class.

Lastly, now, based on your last few questions, I would suggest to get a beginner book and start of from there. Doing things randomly or asking anything would not help. Study, move stepwise and learn.

ConnectionStrings section in web.config.

Sql Authentication

<connectionStrings>
  <add name="ConnectDBString" connectionString="server=SqlServerName;database=DatabaseName;uid=User;password=Password;" />
</connectionStrings>

Windows Authentication

<connectionStrings>
   <add name="ConnectDBString" connectionString="server=SqlServerName;database=DatabaseName;Integrated Security=True" providerName="System.Data.SqlClient" />
 </connectionStrings>

To read the connection string into your code, use the ConfigurationSettings class.

string connStr = ConfigurationManager.ConnectionStrings["ConnectDBString"].ConnectionString;

For further information please go through the following links
1) ConnectionStrings
2) ConnectionStrings

If this would be really helpful to you then don't forgot to Vote and Make Answer as Accepted.

In addition to the other ideas, you can also dynamically build a connection string using the SqlConnectionStringBuilder class.

public class Class1
{

public static string connstring = ConfigurationSettings.AppSettings["Connection"].ToString();
SqlConnection objcon = new SqlConnection(connstring);
SqlCommand objcmd = new SqlCommand();

public bool Opencon()
{
try
{
if (objcon.State == ConnectionState.Closed)
{
objcon.Open();
}
objcmd.Connection = objcon;
return true;
}
catch (Exception ex) { throw new Exception("Error: In Open connesction"); return false; }
}

public bool Closecon()
{
try
{
if (objcon.State == ConnectionState.Open)
{
objcon.Close();
}
objcmd.Dispose();
return true;
}
catch (Exception ex) { throw new Exception("Error: In Close connesction"); return false; }
}


public static int ExecuteQuery(SqlCommand sqlcmd)
{
try
{
Class1 objdc = new Class1();
int affectedrecord = 0;
if (objdc.Opencon() == true)
{
sqlcmd.Connection = objdc.objcon;
affectedrecord = sqlcmd.ExecuteNonQuery();
objdc.Closecon();
objdc = null;
return affectedrecord;
}
else { return affectedrecord; }

}
catch (Exception ex) { throw ex;/* new Exception("Error: In ExecuteNonquery");*/ }

}

public static DataTable Generatedatatable(SqlCommand sqlcmd)
{
try
{
Class1 objdc = new Class1();

if (objdc.Opencon() == true)
{
sqlcmd.Connection = objdc.objcon;
SqlDataReader dr;
DataTable objdt = new DataTable();
dr = sqlcmd.ExecuteReader();
objdt.Load(dr);
objdc.Closecon();
objdc = null;
return objdt;
}
else { return null; }
}
catch (Exception Exception) { throw Exception /*new Exception("Error: In Generatedatatable")*/; }
}
public void RetrieveRecords(DataSet ds, SqlCommand cmd)
{
try
{
cmd.Connection = objcon;
SqlDataAdapter da = new SqlDataAdapter(cmd);
Opencon();
da.Fill(ds);

}
catch (Exception ex)
{
throw ex;
}
finally
{
Closecon();
}
}
public void FillGV(GridView gv, SqlCommand cmd)
{
try
{
DataSet ds = new DataSet();
Opencon();
cmd.Connection =objcon;
RetrieveRecords(ds, cmd);
gv.DataSource = ds;
gv.DataBind();
}
catch (Exception ex)
{
throw ex;
}
finally
{
Closecon();
}
}

public class Class1
{

public static string connstring = ConfigurationSettings.AppSettings["Connection"].ToString();
SqlConnection objcon = new SqlConnection(connstring);
SqlCommand objcmd = new SqlCommand();

public bool Opencon()
{
try
{
if (objcon.State == ConnectionState.Closed)
{
objcon.Open();
}
objcmd.Connection = objcon;
return true;
}
catch (Exception ex) { throw new Exception("Error: In Open connesction"); return false; }
}

public bool Closecon()
{
try
{
if (objcon.State == ConnectionState.Open)
{
objcon.Close();
}
objcmd.Dispose();
return true;
}
catch (Exception ex) { throw new Exception("Error: In Close connesction"); return false; }
}


public static int ExecuteQuery(SqlCommand sqlcmd)
{
try
{
Class1 objdc = new Class1();
int affectedrecord = 0;
if (objdc.Opencon() == true)
{
sqlcmd.Connection = objdc.objcon;
affectedrecord = sqlcmd.ExecuteNonQuery();
objdc.Closecon();
objdc = null;
return affectedrecord;
}
else { return affectedrecord; }

}
catch (Exception ex) { throw ex;/* new Exception("Error: In ExecuteNonquery");*/ }

}

public static DataTable Generatedatatable(SqlCommand sqlcmd)
{
try
{
Class1 objdc = new Class1();

if (objdc.Opencon() == true)
{
sqlcmd.Connection = objdc.objcon;
SqlDataReader dr;
DataTable objdt = new DataTable();
dr = sqlcmd.ExecuteReader();
objdt.Load(dr);
objdc.Closecon();
objdc = null;
return objdt;
}
else { return null; }
}
catch (Exception Exception) { throw Exception /*new Exception("Error: In Generatedatatable")*/; }
}
public void RetrieveRecords(DataSet ds, SqlCommand cmd)
{
try
{
cmd.Connection = objcon;
SqlDataAdapter da = new SqlDataAdapter(cmd);
Opencon();
da.Fill(ds);

}
catch (Exception ex)
{
throw ex;
}
finally
{
Closecon();
}
}
public void FillGV(GridView gv, SqlCommand cmd)
{
try
{
DataSet ds = new DataSet();
Opencon();
cmd.Connection =objcon;
RetrieveRecords(ds, cmd);
gv.DataSource = ds;
gv.DataBind();
}
catch (Exception ex)
{
throw ex;
}
finally
{
Closecon();
}
}

<configuration>
<configsections>

<connectionstrings>
<add name="StockSoftware.Properties.Settings.StockSoftwareConnectionString" connectionstring="Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\PetPlastDB.accdb">
providerName="System.Data.OleDb" />

<appsettings>
<add key="petplast" value="Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\\PetPlastDB.accdb">