Check if Base64 string is Valid or not using C# and VB.Net in ASP.Net

Hi IamAzhar,

Use below method to check valid base64 string.

C#

public bool IsBase64String(string base64)
{
    base64 = base64.Trim();
    return (base64.Length % 4 == 0) && Regex.IsMatch(base64, @"^[a-zA-Z0-9\+/]*={0,3}$", RegexOptions.None);
}

VB.Net

Public Function IsBase64String(ByVal base64 As String) As Boolean
    base64 = base64.Trim()
    Return (base64.Length Mod 4 = 0) AndAlso Regex.IsMatch(base64, "^[a-zA-Z0-9\+/]*={0,3}$", RegexOptions.None)
End Function

Or you can use try catch.

C#

public static bool TryGetFromBase64String(string input, out byte[] output)
{
    output = null;
    try
    {
        output = Convert.FromBase64String(input);
        return true;
    }
    catch (FormatException ex)
    {
        return false;
    }
}

VB.Net

Public Shared Function TryGetFromBase64String(ByVal input As String, <Out> ByRef output As Byte()) As Boolean
    output = Nothing
    Try
        output = Convert.FromBase64String(input)
        Return True
    Catch ex As FormatException
        Return False
    End Try
End Function

Or refer below link.

https://stackoverflow.com/questions/6309379/how-to-check-for-a-valid-base64-encoded-string