Call Stored Procedure using Web API with Nullable Parameters using C# in ASP.Net

You need to pass Nullable Parameters to the Procedure when select only start and end date.

Check with below code.

[HttpGet]
[ActionName("getStudentDetails")]
public string getStudentDetails(string StartDate, string EndDate, int StudentId, int DeptId, int CollegeId, int SectionId)
{
    SqlConnection myConnection = new SqlConnection();
    myConnection.ConnectionString = ConfigurationManager.ConnectionStrings[""].ConnectionString;
    SqlCommand sqlCmd = new SqlCommand();
    sqlCmd.CommandType = CommandType.StoredProcedure;
    sqlCmd.CommandText = "STUDENT_DETAILS";
    sqlCmd.Connection = myConnection;
    sqlCmd.Parameters.AddWithValue("@StartDate", StartDate);
    sqlCmd.Parameters.AddWithValue("@EndDate", EndDate);
    sqlCmd.Parameters.AddWithValue("@StudentId", StudentId > 0 ? StudentId : (object)DBNull.Value);
    sqlCmd.Parameters.AddWithValue("@DeptId", DeptId > 0 ? DeptId : (object)DBNull.Value);
    sqlCmd.Parameters.AddWithValue("@CollegeId", CollegeId > 0 ? CollegeId : (object)DBNull.Value);
    sqlCmd.Parameters.AddWithValue("@SectionId", SectionId > 0 ? SectionId : (object)DBNull.Value);
    SqlDataAdapter sda = new SqlDataAdapter(sqlCmd);
    DataSet ds = new DataSet();
    string jsonString = string.Empty;
    myConnection.Open();
    sda.Fill(ds);
    myConnection.Close();
    jsonString = JsonConvert.SerializeObject(ds.Tables[0]);
    return jsonString;
}

And in where condition check the columns with nullable parameters.

ALTER PROCEDURE [dbo].[STUDENT_DETAILS]
    @STARTDATE DATETIME = NULL,
    @ENDDATE DATETIME = NULL,
    @STUDENTID INT = NULL,
    @DEPTID INT = NULL,
    @COLLEGEID INT = NULL,
    @SECTIONID INT = NULL
AS
BEGIN
	SELECT * FROM Table
	WHERE BDate BETWEEN @STARTDATE AND @ENDDATE
	AND (StudentId = @STUDENTID OR @STUDENTID IS NULL)
	AND (DeptId = @DEPTID OR @DEPTID IS NULL)
	AND (CollegeId = @COLLEGEID OR @COLLEGEID IS NULL)
	AND (SectionId = @SECTIONID OR @SECTIONID IS NULL)
END

[HttpGet]
[ActionName("getStudentDetails")]
public string getStudentDetails(string StartDate, string EndDate, int? StudentId = null, int? DeptId = null, int? CollegeId = null, int? SectionId = null)
{
    SqlConnection myConnection = new SqlConnection();
    myConnection.ConnectionString = ConfigurationManager.ConnectionStrings[""].ConnectionString;
    SqlCommand sqlCmd = new SqlCommand();
    sqlCmd.CommandType = CommandType.StoredProcedure;
    sqlCmd.CommandText = "STUDENT_DETAILS";
    sqlCmd.Connection = myConnection;
    sqlCmd.Parameters.AddWithValue("@StartDate", StartDate);
    sqlCmd.Parameters.AddWithValue("@EndDate", EndDate);
    sqlCmd.Parameters.AddWithValue("@StudentId", StudentId);
    sqlCmd.Parameters.AddWithValue("@DeptId", DeptId);
    sqlCmd.Parameters.AddWithValue("@CollegeId", CollegeId);
    sqlCmd.Parameters.AddWithValue("@SectionId", SectionId);
    SqlDataAdapter sda = new SqlDataAdapter(sqlCmd);
    DataSet ds = new DataSet();
    string jsonString = string.Empty;
    myConnection.Open();
    sda.Fill(ds);
    myConnection.Close();
    jsonString = JsonConvert.SerializeObject(ds.Tables[0]);
    return jsonString;
}

Instead of using OR condition in sql, we can use COALESCE function

DeptId = COALESCE(@DEPTID, DEPTID)