Azure SQL Database Application

Use token-based authentication on your application for simplicity.

public async Task<string> GetAccessTokenAsync(string clientId, string clientSecret, string authority, string resource, string scope)
{
    var authContext = new AuthenticationContext(authority, TokenCache.DefaultShared);
    var clientCred = new ClientCredential(clientId, clientSecret);
    var result = await authContext.AcquireTokenAsync(resource, clientCred);

    if (result == null)
    {
        throw new InvalidOperationException("Could not get token");
    }

    return result.AccessToken;
}

Creating a SQL Connection using token.

public async Task<SqlConnection> GetSqlConnectionAsync(string tenantId, string clientId, string clientSecret, string dbServer, string dbName)
{
    var authority = string.Format("https://login.windows.net/{0}", tenantId);
    var resource = "https://database.windows.net/";
    var scope = "";
    var token = await GetTokenAsync(clientId, clientSecret, authority, resource, scope);

    var builder = new SqlConnectionStringBuilder();
    builder["Data Source"] = $"{dbServer}.database.windows.net";
    builder["Initial Catalog"] = dbName;
    builder["Connect Timeout"] = 30;
    builder["Persist Security Info"] = false;
    builder["TrustServerCertificate"] = false;
    builder["Encrypt"] = true;
    builder["MultipleActiveResultSets"] = false;

    var con = new SqlConnection(builder.ToString());
    con.AccessToken = token;
    return con;
}

You don't even need to worry about token expiration since AzureServiceTokenProvider takes care of caching.

Learn about it on this article.

Did you think about the Azure firewall? To add the client IP or IP address range to limit the access of Azure SQL database.

Set server firewall on portal:

Firewall settings: to add the client IP or IP address range to provides the access the database.

Fore more details, please see:

1. Azure SQL Database and SQL Data Warehouse IP firewall rules.
2. Create a server-level IP firewall rule using Azure Portal.

Hope this helps.