How to Correctly Call a SQL Server User-Defined Function (UDF) from C#?

Calling SQL Defined Functions in C# Using Inline SQL

In SQL Server, user-defined functions (UDFs) can enhance data manipulation and analysis capabilities. To invoke a scalar UDF from C# code, it's important to employ the correct approach.

Consider the following TSQL scalar UDF:

create function TCupom (@cupom int)
returns float
as
begin
    declare @Tcu float;
    select @Tcu = sum (total) from alteraca2 where pedido = @cupom 
    if (@tcu is  null)
        set @tcu = 0;
    return @tcu;
end

To call this function in C# code, one may attempt to use a stored procedure-like syntax:

public void TotalCupom(int cupom)
{ 
    float SAIDA;           
    SqlDataAdapter da2 = new SqlDataAdapter();

    if (conex1.State == ConnectionState.Closed)
    { 
        conex1.Open();
    }

    SqlCommand Totalf = new SqlCommand("Tcupom", conex1);
    SqlParameter code1 = new SqlParameter("@code", SqlDbType.Int);
    code1.Value = cupom ;
    Totalf.CommandType = CommandType.StoredProcedure ;
    SAIDA = Totalf.ExecuteScalar();

    return SAIDA;
}

However, this approach is incorrect. To invoke a UDF, it's necessary to use inline SQL within a SqlCommand object:

SqlCommand Totalf = new SqlCommand("SELECT dbo.Tcupom(@code)", conex1);

It's crucial to remove the CommandType property, as UDFs are not stored procedures.

The modified code would appear as follows:

public void TotalCupom(int cupom)
{ 
    float SAIDA;           
    SqlDataAdapter da2 = new SqlDataAdapter();
    if (conex1.State == ConnectionState.Closed)
    {
        conex1.Open();
    }
    SqlCommand Totalf = new SqlCommand("SELECT dbo.Tcupom(@code)", conex1);
    SqlParameter code1 = new SqlParameter("@code", SqlDbType.Int);
    code1.Value = cupom;
    SAIDA = Totalf.ExecuteScalar();

    return SAIDA;
}

By employing inline SQL to call the UDF, developers can effectively harness the power of user-defined functions in their C# applications.

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