Details

Create a TokenService class that takes an IConfiguration and a UserManager<ApplicationUser>

Create a private GenerateAccessToken method

private string GenerateAccessToken(ApplicationUser user)
   {
       var key = config.GetValue<string>("Jwt:Key");
       var issuer = config.GetValue<string>("Jwt:Issuer");
       var audience = config.GetValue<string>("Jwt:Audience");
       
       // Ensure configuration values are not null or empty
       if (string.IsNullOrEmpty(key) ||
           string.IsNullOrEmpty(issuer) ||
           string.IsNullOrEmpty(audience))
       {
           throw new InvalidOperationException("JWT configuration values are missing.");
       }
       
       var securityKey = new SymmetricSecurityKey(
           Encoding.UTF8.GetBytes(key));
       var credentials = new SigningCredentials(
           securityKey, SecurityAlgorithms.HmacSha256);
       var claims = new List<Claim>
       {
           new (ClaimTypes.NameIdentifier, user.Id),
           new (ClaimTypes.Email, user.Email ?? string.Empty)
       };
       // Generate token
       var token = new JwtSecurityToken(
           issuer: issuer,
           audience: audience,
           claims: claims,
           notBefore: DateTime.UtcNow,
           expires: DateTime.UtcNow.AddDays(1),
           signingCredentials: credentials);
       var tokenString = new JwtSecurityTokenHandler().WriteToken(token);
       return tokenString;
   }

Create a private GenerateRefreshToken method

private static string GenerateRefreshToken()
   {
       var randomNumber = new byte[32];
       using var rng = RandomNumberGenerator.Create();
       rng.GetBytes(randomNumber);
       return Convert.ToBase64String(randomNumber);
   }

Then create a public GenerateTokensAsync method and call both private methods

public async Task<BaseResponse> GenerateTokensAsync(ApplicationUser user)
   {
       var accessToken = GenerateAccessToken(user);
       var refreshToken = GenerateRefreshToken();
       user.RefreshToken = refreshToken;
       user.RefreshTokenExpiryTime = DateTime.UtcNow.AddDays(30);
       var result = await userManager.UpdateAsync(user);
       if (result.Succeeded is false)
       {
           return new BaseResponse()
           {
               Succeeded = false,
               Message = "Failed to generate tokens.",
               Data = new { result.Errors }
           };
       }
       return new BaseResponse()
       {
           Succeeded = true,
           Message = "Successfully generated tokens.",
           Data = new
           {
               accessToken,
               refreshToken
           }
       };
   }

Then register the service with dependency injection

services.AddScoped<TokenService>();

Then use it like any other service!