Insert data into DB from EF using composite key (primary)

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C#

EF6.0

Data is not inserting into DB, How handle composite primary key, where one PK is set to auto-increment. Here not entering any data into Id column , but it throws following exception .

"Cannot insert explicit value for identity column in table 'TableName' when IDENTITY_INSERT is set to OFF."

What I have tried:

Table 
Id int primary key (auto increment)
SrNo varchar(6) primary key
Name varchar(10)

<pre lang="c#">   this.HasKey(m => new { m.Id, m.SrNo });

public bool Add(BusinessEntity mod)
                   {  _itemWarehouse.Insert(mod);   }

taking data from WinForm where
Id=0 (didn't set, default), SrNo = X12 , Name = ABC

But not entering data into DB , throwing exception

"Cannot insert explicit value for identity column in table 'TableName' when IDENTITY_INSERT is set to OFF."